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UBC Theses and Dissertations

Interactions and coexistence of species in pasture community evolution 1983

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INTERACTIONS AND COEXISTENCE OF SPECIES IN PASTURE COMMUNITY EVOLUTION by LONNIE WILLIAM AARSSEN B.Sc.(Hons.), The U n i v e r s i t y of Western O n t a r i o , 1978 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF BOTANY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA January 1983 (c) Lonnie W i l l i a m Aarssen In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree a t the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood t h a t copying o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of BOTANY The U n i v e r s i t y o f B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date January 16, 1983 DE-6 (3/81) "IT IS GENERALLY ACCEPTED THAT IN ORDER TO COEXIST MORE THAN TRANSIENTLY SPECIES MUST DIFFER - THEY MUST SHOW NICHE SEPARATION. IF SPECIES ARE TOO SIMILAR ALL BUT ONE WILL BE ELIMINATED IN COMPETITION " NEWMAN (1982) "IF NOTHING IN BIOLOGY HAS MEANING EXCEPT IN THE LIGHT OF EVOLUTION AND IF EVOLUTION IS ABOUT INDIVIDUALS AND THEIR DESCENDENTS - I.E. FITNESS - WE SHOULD NOT EXPECT TO REACH ANY DEPTH OF UNDERSTANDING FROM STUDIES THAT ARE BASED AT THE LEVEL OF THE SUPER- ' INDIVIDUAL., •. . . ., - WHAT. WE SEE AS - 'THE ORGANIZED BEHAVIOUR OF SYSTEMS IS THE RESULT OF THE FATE OF INDIVIDUALS" '. HARPER (1977a) ABSTRACT Recent s t u d i e s have demonstrated i n t r a s p e c i f i c d i f f e r e n t i a t i o n i n p l a n t s a s s o c i a t e d with s p e c i a l i z a t i o n i n response to b i o t i c i n t e r a c t i o n s (e.g. c o m p e t i t i o n ) , but the s e l e c t i o n mechanisms i n v o l v e d remain l a r g e l y undetermined. The common assumption that s p e c i e s c o e x i s t e n c e i n nature can be g e n e r a l l y e x p l a i n e d by processes of n a t u r a l s e l e c t i o n f o r niche divergence does not have strong e m p i r i c a l support f o r p l a n t s . T h i s problem i s addressed i n a study of v e g e t a t i o n p a t t e r n s and s p e c i e s i n t e r a c t i o n s i n three d i f f e r e n t aged p a s t u r e s . O r d i n a t i o n of t i m e - s e r i e s percentage cover surveys showed a t r e n d of i n c r e a s i n g community 'constancy' i n o l d e r pastures and suggested a developmental r e l a t i o n s h i p -amongst the p a s t u r e s . S o i l a n a l y ses showed l i t t l e c o r r e l a t i o n with s p e c i e s cover. Temporal p a t t e r n s of f i n e - s c a l e a s s o c i a t i o n between s p e c i e s was s t u d i e d using c o n t a c t sampling. I n t e r s p e c i f i c a s s o c i a t i o n s i n younger communities were predominantly temporary in nature, and o l d e r . communities had more a s s o c i a t i o n s which p e r s i s t e d e s s e n t i a l l y unchanged. T h i s - data formed the b a s i s of a q u a l i t a t i v e model of pasture community e v o l u t i o n which a t t r i b u t e s within-community temporal changes to the s e l e c t i v e f o r c e s a c c r u i n g from b i o t i c i n t e r a c t i o n s . Competition experiments were set up f o r i n v e s t i g a t i n g r e c i p r o c a l responses between c l o n e s of i n d i v i d u a l s which were in immediate p r o x i m i t y to one another as a c t u a l neighbours i n the f i e l d s . For each of the 3 p a s t u r e s , f i v e of the most abundant s p e c i e s were used in a d i a l l e l design and three d i f f e r e n t i v s p e c i e s p a i r s were s t u d i e d in a replacement s e r i e s d e s i g n . R e s u l t s suggested that c o m p e t i t i v e r e l a t i o n s between p a r t i c u l a r s p e c i e s may change with i n c r e a s i n g pasture age e i t h e r i n the d i r e c t i o n of i n c r e a s i n g niche d i f f e r e n t i a t i o n , more balanced c o m p e t i t i v e a b i l i t i e s , or towards c o m p e t i t i v e e x c l u s i o n . Samples of Lolium perenne and T r i f o l i u m repens c l o n e s c o l l e c t e d as neighbouring p a i r s from d i f f e r e n t l o c a l i t i e s in the o l d e s t pasture were t e s t e d f o r t h e i r a b i l i t y to grow in the presence of each other i n a r e c i p r o c a l t r a n s p l a n t d e s i g n . N a t u r a l neighbouring genet p a i r s had the most e q u i t a b l e component c o n t r i b u t i o n to the t o t a l y i e l d ( i . e . h i g h e s t combining a b i l i t y ) but d i d not d i f f e r i n t o t a l y i e l d from p a i r s of non- n a t u r a l neighbours. R e s u l t s from these s t u d i e s suggest that s e l e c t i o n i n response to competition and other i n t e r a c t i o n s among neighbours (e.g. beneficence) may r e s u l t i n two a l t e r n a t i v e types of 'combining a b i l i t y ' i n p l a n t s : 1) e c o l o g i c a l combining a b i l i t y (niche d i f f e r e n t i a t i o n ) , and 2) competitive combining a b i l i t y (balanced c o m p e t i t i v e a b i l i t i e s ) . Most evidence was found f o r 2). In t h i s mechanism, s e l e c t i o n reduces the d i f f e r e n t i a l i n c o m p e t i t i v e a b i l i t i e s of i n f e r i o r and s u p e r i o r components for resources on which they both make demands. These f i n d i n g s are d i s c u s s e d i n r e l a t i o n to contemporary t h e o r e t i c a l c o n s i d e r a t i o n s of n a t u r a l s e l e c t i o n o p e r a t i n g i n systems of c o m p e t i t i o n . A general e v o l u t i o n a r y theory of c o e x i s t e n c e i s o f f e r e d based on a proposed d i s t i n c t i o n between the concepts of fundamental niche and c o m p e t i t i v e a b i l i t y . V TABLE OF CONTENTS ABSTRACT i i i TABLE OF CONTENTS v LIST OF TABLES v i i i LIST OF FIGURES x i ACKNOWLEDGEMENTS x i v CHAPTER 1. GENERAL INTRODUCTION 1 PROBLEM 2 TERMS AND CONCEPTS . 9 A) INTERACTION AND COEXISTENCE 9 B) FUNDAMENTAL NICHE REQUIREMENTS 11 C) RELATIVE COMPETITIVE ABILITY 14 O f f e n s i v e C h a r a c t e r i s t i c s C o n f e r r i n g Competitive A b i l i t y 15 Defensive C h a r a c t e r i s t i c s C o n f e r r i n g Competitive A b i l i t y 18 PROGRAMME 20 CHAPTER 2. THE STUDY SITE: VARIABLES AND PATTERNS ..22 INTRODUCTION ........ 23 THE GEOLOGY 24 THE CLIMATE 29 THE SPECIES 30 A) PATTERNS IN THE VEGATATION 31 Methods 32 R e s u l t s 33 B) COLLECTION AND PROPAGATION OF CLONES 42 v i SOIL SAMPLING AND ANALYSES 45 Methods 45 Re s u l t s 46 DISCUSSION 55 CHAPTER 3. A QUALITATIVE MODEL FOR PASTURE COMMUNITY EVOLUTION: PRELIMINARY EVIDENCE FOR SPECIES INTERACTIONS AND COEXISTENCE USING CONTACT SAMPLING 64 INTRODUCTION 65 METHODS 69 Veg e t a t i o n Sampling 69 Data A n a l y s i s 71 RESULTS 75 DISCUSSION 93 CHAPTER 4. COMPETITIVE RELATIONS IN DIFFERENT AGED PASTURES: A MIXTURE DIALLEL OF FIVE SPECIES 106 INTRODUCTION ..107 METHODS 111 RESULTS 114 DISCUSSION . . . . . ..... ; . ....... . . ... ..V. 126 CHAPTER 5. COMPETITIVE RELATIONS IN DIFFERENT AGED PASTURES: SUBSTITUTIVE REPLACEMENT SERIES MODEL 132 INTRODUCTION 133 THEORY 135 METHODS .140 RESULTS AND DISCUSSION 143 v i i CHAPTER 6. BIOTIC SPECIALIZATION AT THE GENOTYPE LEVEL: RECIPROCAL PHYTOMETER TRANSPLANTS AMONGST FOUR NATURAL NEIGHBOURING GENET PAIRS OF LOLIUM PERENNE AND TRIFOLIUM REPENS 158 INTRODUCTION 159 METHODS 164 Experimental Design 164 Data A n a l y s i s 168 RESULTS 171 DISCUSSION 182 CHAPTER 7. GENERAL DISCUSSION 194 PROSPECTUS 195 COMPONENTS OF A GENERALIZED COEXISTENCE THEORY 197 A) Gaussian Coexistence 197 B) Coexistence by s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y (niche d i f f e r e n t i a t i o n ) 201 C) Coexistence by s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y ( c o e v o l u t i o n ) 203 OTHER MECHANISMS FOR COMPETITIVE BALANCE 210 1) Non-evolutionary mechanisms 210 2) Frequency-dependent -select ion v... . . ; . . ... 211 3) D i f f e r e n t . l i m i t i n g f a c t o r s 214 NICHE DIFFERENTIATION VERSUS COEVOLUTION OF COMPETITORS: ALTERNATIVE EVOLUTIONARY SOLUTIONS FOR SPECIES COEXISTENCE 218 CONCLUSIONS 229 LITERATURE CITED 231 v i i i LIST OF TABLES TABLE 1. P o s s i b l e c h a r a c t e r s which confer c o m p e t i t i v e a b i l i t y in p l a n t s i n co n t e x t s of a) r e l a t i v e e x p l o i t a t i o n a b i l i t y , and b) r e l a t i v e i n t e r f e r e n c e a b i l i t y 16 TABLE 2. D e t a i l s on the three study f i e l d s 23 TABLE 3. The monthly d i s t r i b u t i o n s of a) r a i n f a l l (cm), b) s n o w f a l l (cm) and c) t o t a l p r e c i p i t a t i o n •(cm) 29 TABLE 4. The monthly d i s t r i b u t i o n s o f : a) mean d a i l y temperature ( C), b) mean d a i l y maximum temperature ( C) , c) mean d a i l y minimum temperature ( C); d) number of days with f r o s t ( d a i l y minimum below 0 C) 30 TABLE 5. A s p e c i e s l i s t f o r the study s i t e 31 TABLE 6. Species composition of the 'High-land' forage mixture used i n sowing the three p a s t u r e s . 32 TABLE 7. Product moment c o r r e l a t i o n c o e f f i c i e n t s f o r the 9 measured s o i l v a r i a b l e s with quadrat percentage cover f o r 10 of the most abundant s p e c i e s o v e r a l l at the study s i t e and with a x i s scores f o r the f i r s t 3 axes of the o r d i n a t i o n of 20 s e l e c t e d quadrats. . . .......... . . .56 TABLE 8. Simpson's d i v e r s i t y indexes (D) c a l c u l a t e d f o r each survey based on t o t a l percentage cover of each s p e c i e s per f i e l d 62 TABLE 9. Count matrix generated by c o n t a c t sampling 72 TABLE 10. Contingency t a b l e f o r ( i , j ) i n t e r a c t i o n 72 TABLE 11. Summary of s i g n i f i c a n t (P<0.05) i n d i v i d u a l p a i r w i s e a s s o c i a t i o n s i n the three pastures i n each survey 76 ix TABLE 12. Numbers of s i g n i f i c a n t temporary, seasonal, and s t a b l e a s s o c i a t i o n s (both p o s i t i v e and negative) d e t e c t e d i n the three d i f f e r e n t aged pastures , 94 TABLE 13. 5 x 5 ma t r i c e s of p e r - s p e c i e s y i e l d s (g) (means of 3 r e p l i c a t e s ) from the mixture d i a l l e l i n v o l v i n g 5 s p e c i e s p l a n t e d as ramets. 1 1 5 TABLE 14. A summary of s i g n i f i c a n t (P<0.05) trends i n combining a b i l i t y indexes (CA) and t o t a l y i e l d s from the mixture d i a l l e l f o r d i f f e r e n t genet p a i r types as the age of the pasture from which they were c o l l e c t e d i n c r e a s e s . 1 24 TABLE 15. N u t r i e n t composition of p o t t i n g mix used f o r the c o m p e t i t i o n experiment between d i f f e r e n t genets of Lolium perenne and T r i f o l i u m repens. . . 1 65 TABLE 16, Tabular model v a r i a n c e . used i n the a n a l y s i s of 1 70 TABLE 17. Y i e l d s (g) of phytometers of T r i f o l i u m repens (Tx), each c o l l e c t e d with a n a t u r a l neighbouring genet of Lolium perenne (Lx) from four s i t e s i n the 1939 pasture and p l a n t e d i n a l l combinations type of c l o v e r type and grass 1 72 TABLE 18. Y i e l d s (g) of phytometers of Lolium perenne (Lx), each c o l l e c t e d with a n a t u r a l neighbouring genet of : T r i f o l i u r n , repens (Tx) • from four s i t e s i n the 1939 pasture and p l a n t e d i n a l l combinations of c l o v e r type and grass type 173 TABLE 19. T o t a l combined y i e l d s (g) of phytometers of Lolium perenne (L) and Tr i f o l i u m repens (T) c o l l e c t e d as n a t u r a l neighbouring p a i r s from four s i t e s i n the 1939 pasture and p l a n t e d in a l l combinations of grass type and c l o v e r type 1 74 X TABLE 20. Combining a b i l i t y indexes f o r genet p a i r s of Lolium perenne (L) and T r i f o l i u m repens (T) c o l l e c t e d as n a t u r a l neighbouring p a i r s from four s i t e s i n the 1939 pasture and p l a n t e d i n a l l combinations of grass type and c l o v e r type 175 TABLE 2 1 . A summary of i n t e r p r e t a t i o n s of c o m p e t i t i v e r e l a t i o n s between s p e c i e s r e p o r t e d from the experimental i n v e s t i g a t i o n s . 208 x i LIST OF FIGURES FIGURE 1. Contour maps of the study f i e l d s . 25 FIGURE 2. T o t a l percentage cover of the 14 most abundant species present i n the three pasture communities over the 33-month sampling p e r i o d 34 FIGURE 3. The 10 most abundant s p e c i e s present i n each study f i e l d based on mean percentage cover over the study p e r i o d . 40 FIGURE 4. O r d i n a t i o n r e s u l t s of the 27 surveys 43 FIGURE 5. Mean values f o r chemical analyses of s o i l from 20 s e l e c t e d stands from each pasture (4, 23 and 42 years old) 47 FIGURE 6. D i s t r i b u t i o n a l p a t t e r n s of the 9 s o i l v a r i a b l e s on the a x i s 2 versus a x i s 1 plane of the stand o r d i n a t i o n of 20 s e l e c t e d quadrats from each p a s t u r e . 49 FIGURE 7. S t a b l e p a i r w i s e a s s o c i a t i o n s ( p o s i t i v e s o l i d l i n e s , and negative - broken l i n e s ) d e t e c t e d i n a) the 1958 pasture, and b) the 1939 pasture 86 FIGURE 8. Course l i n e s showing the degree of a s s o c i a t i o n f o r s e l e c t e d i n d i v i d u a l s p e c i e s p a i r s versus the number of years s i n c e the sampled f i e l d was sown 88 FIGURE 9. A q u a l i t a t i v e model f o r pasture community e v o l u t i o n 98 FIGURE 10. P l a n t i n g arrangement i n the mixture d i a l l e l showing p o s i t i o n s f o r 25 ramets of each of two spec i e s 112 x i i FIGURE 11. I n d i v i d u a l y i e l d s , t o t a l y i e l d s and combining a b i l i t y indexes (CA) f o r each genet p a i r type c o l l e c t e d from d i f f e r e n t aged pastures (0, 2, 21 and 40 years) and grown i n co m p e t i t i o n i n a mixture d i a l l e l 118 FIGURE 12. R a t i o diagrams i l l u s t r a t i n g 5 p o s s i b l e outcomes from a replacement s e r i e s experiment r e f l e c t i n g 5 d i f f e r e n t types of c o m p e t i t i v e r e l a t i o n s h i p between s p e c i e s i and j . 136 FIGURE 13. P l a n t i n g arrangement used f o r 13 ramets i n the replacement s e r i e s 141 FIGURE 14. R a t i o diagrams f o r the experimental r e s u l t s . ..144 FIGURE 15. Ra t i o diagrams showing t h e o r e t i c a l trends f o r e v o l u t i o n a r y changes i n c o m p e t i t i v e r e l a t i o n s between two s p e c i e s ( i and j ) durin g d i f f e r e n t stages (1, 2 and 3) of n a t u r a l s e l e c t i o n . 148 FIGURE 16. R a t i o diagrams f o r immediately adjacent genet p a i r s from the three d i f f e r e n t aged pastures (2, 21 and 40 yrs) superimposed on the same graph 151 FIGURE 17. P l a n t i n g arrangement f o r pots c o n t a i n i n g d i f f e r e n t genet combinations of Lolium perenne (L) and Tr i f o l ium repens (T) 166 FIGURE 18. Y i e l d s of phytometers of Lolium perenne (L) and T r i f o l i u m repens (T) when grown together in d i f f e r e n t combinations of genet types 176 FIGURE 19. Combining a b i l i t y indexes and t o t a l y i e l d s f o r d i f f e r e n t genet type combinations of L o l ium perenne and T r i f olium repens 179 FIGURE 20. R e l a t i o n s h i p between t o t a l y i e l d and combining a b i l i t y index f o r d i f f e r e n t genet type combinations of Lolium perenne and T r i f o l i u m repens 183 XI 1 1 FIGURE 21. The schematic s t r u c t u r e of a general theory of s p e c i e s c o e x i s t e n c e i n contexts where competition i s an important f o r c e of n a t u r a l s e l e c t i o n 1 98 x i v ACKNOWLEDGEMENTS I am most deeply g r a t e f u l to Dr. Roy Turkington f o r h i s gr a c i o u s support and e x t e n s i v e enthusiasm i n the s u p e r v i s i o n of t h i s r e s e a r c h . Much debt i s owed to Dr. P i e t de Jong f o r h i s a t t e n t i v e and i n v a l u a b l e counsel i n s t a t i s t i c a l matters. A p p r e c i a t i o n i s expressed to Dr. Jack Maze and to Dr. W.E. N e i l l for, i n s p i r i n g d i s c u s s i o n s and f o r t h e i r i n t e r e s t i n my re s e a r c h . I wish to acknowledge t e c h n i c a l a s s i s t a n c e from Brenda K i l g r e n , Melanie M a d i l l , C a r o l George, Diane S c o t t , Jan Evans, J u l i e Downum, Angela Chen, and Elena K l e i n e . Thanks i s due to Helene Contant and Joop van Velzen f o r he l p i n t r a n s l a t i n g f o r e i g n l i t e r a t u r e . I a l s o thank the s t a f f of the U.B.C. Botany Garden Nursery f o r t h e i r care i n the maintenance of glasshouse experiments. F i n a n c i a l support through postgraduate s c h o l a r s h i p from the Na t u r a l Sciences and En g i n e e r i n g Research C o u n c i l of Canada i s g r a t e f u l l y acknowlegedv F i n a l l y , I extend immeasurable g r a t i t u d e to Mr. W i l l i a m Chard and Ms. Mary Chard f o r t h e i r c o r d i a l welcome to conduct t h i s research on t h e i r p r o p e r t y , and to whom t h i s t h e s i s i s a f f e c t i o n a t e l y d e d i c a t e d . 1 CHAPTER 1 GENERAL INTRODUCTION 2 PROBLEM The c o e x i s t e n c e of s p e c i e s which i n t e r a c t c o m p e t i t i v e l y has long i n t e r e s t e d e c o l o g i s t s and the q u e s t i o n of what permits so many competitors to c o e x i s t i n some communities has continued l a r g e l y u nresolved (Hutchinson 1959, 1961, M i l l e r 1969, Grubb 1977, Werner 1979). The n o t i o n of c o e x i s t e n c e has been so coupled with the concept of co m p e t i t i o n that i t may be a p t l y d e f i n e d as the 'absence of c o m p e t i t i v e e x c l u s i o n ' , much l i k e h e a l t h i s the absence of d i s e a s e . In g e n e r a l , e f f o r t s have been concerned with e s t a b l i s h i n g what f a c t o r s or mechanisms prevent c o m p e t i t i v e e x c l u s i o n of one s p e c i e s by another i n a resource l i m i t e d environment. Gause's p r i n c i p l e has formed the t h e o r e t i c a l b a s i s and has been regarded as "... perhaps the only s p e c i f i c p r i n c i p l e or law of nature ever to be proposed i n ecology" (Vandermeer 1972a). The common no t i o n of i d e n t i c a l e c o l o g i e s reduces the p r i n c i p l e to a ' t r i t e maxim' (Cole 1960) but recent p e r s p e c t i v e s have abandoned t h i s and are more concerned with ..what Is the l i m i t of niche o v e r l a p beyond ] which" co m p e t i t i v e e x c l u s i o n of;one of the s p e c i e s i s . imminent. ,This more o p e r a t i o n a l view embodies the theory of s p e c i e s packing and ' l i m i t i n g s i m i l a r i t y of s p e c i e s ' (MacArthur & Le v i n s 1967, MacArthur & Wilson 1967, MacArthur 1972, Newman 1982) and i n t e r e s t i n g l y , r e c a p i t u l a t e s one of the e a r l i e s t statements of the c o m p e t i t i v e e x c l u s i o n p r i n c i p l e : "Two s p e c i e s of approximately the same food h a b i t s are not l i k e l y to remain long evenly balanced i n numbers i n the same r e g i o n . One w i l l crowd out the other" ( G r i n n e l l 1904) ( i t a l i c s added). 3 Most of the support f o r Gause's p r i n c i p l e comes from o b s e r v a t i o n s of niche d i f f e r e n c e s in animals (e.g. see e x t e n s i v e review by Schoener (1974)). The idea of the 'food niche' of animals was c e n t r a l to the e a r l y t h e o r e t i c a l development of the niche concept. Plant e c o l o g i s t s however must de a l with the f a c t t h a t p l a n t s have v i r t u a l l y nothing comparable to the food niche (Harper 1977b). The c o e x i s t e n c e of animals can be e x p l a i n e d i n p a r t by t h e i r m o b i l i t y , but more importantly by the d i v e r s i t y of foods a v a i l a b l e to them, e s p e c i a l l y p l a n t s . We are s t i l l l e f t however with the problem of e x p l a i n i n g p l a n t s p e c i e s c o e x i s t e n c e . W i l l s o n (1973) and Antonovics (1978) have drawn a t t e n t i o n to the s c a r c i t y of c l e a r e m p i r i c a l evidence f o r within-community niche d i f f e r e n t i a t i o n i n p l a n t s . T h i s i s p a r t i c u l a r l y t rue i n herbaceous p l a n t communities. Some recent s t u d i e s have shown that c o - o c c u r r i n g p l a n t s p e c i e s may show d i f f e r e n c e s , f o r example, in depth and placement of r o o t s (Wieland & Bazzaz 1975, P a r r i s h & Bazzaz 1976, Yeaton et a l . 1977, Berendse 1979, 1981a, F i t t e r 1982), d i f f e r e n c e s i n response to .microtopography - (Brattori 1976, S i l v e r t o w n 1981), p a r t i t i o n i n g of p o l l i n a t o r s (Reader 1975, P a r r i s h & Bazzaz. 1 9 7 8 , 1979, Waser 1978) and s e p a r a t i o n along s o i l moisture g r a d i e n t s ( P i c k e t t & Bazzaz 1976, 1978, Werner & P i a t t 1976, P i a t t & Weis 1977). Yet, because higher p l a n t s are r e l a t i v e l y immobile, lack any r e a l c h o i c e i n energy supply , and make demands on e s s e n t i a l l y the same resources (of l i g h t , water and n u t r i e n t s ) (Harper 1968), i t i s d i f f i c u l t to imagine how there c o u l d not be c o n s i d e r a b l e niche o v e r l a p and enormous o p p o r t u n i t y f o r competition i n many p l a n t communities. T h i s has r a i s e d s e r i o u s 4 doubts concerning any predominant or widespread importance of niche d i f f e r e n t i a t i o n as a mechanism f o r c o e x i s t e n c e of p l a n t s (Grubb 1977, Fagerstrom & Agren 1979, P i c k e t t 1980). The study of s p e c i e s c o e x i s t e n c e i s an e v o l u t i o n a r y problem and i s at the i n t e r f a c e of p o p u l a t i o n and community ecology. I t concerns p o p u l a t i o n i n t e r a c t i o n s and community o r g a n i z a t i o n and d i v e r s i t y . I t embodies a 'Darwinian view' of nature which p l a c e s strong emphasis on the s e l e c t i o n p r e s s u r e s which accrue from c o m p e t i t i v e i n t e r a c t i o n s as opposed to a 'Wallacian view' which emphasizes more the s e l e c t i o n p r e s s u r e s from a b i o t i c f a c t o r s (Harper 1977b). Darwin r e f e r s to the p r a c t i c e of some in making, .... the deeply-seated e r r o r of c o n s i d e r i n g the p h y s i c a l c o n d i t i o n s of a country as the most important f o r i t s i n h a b i t a n t s ; whereas i t cannot, I t h i n k , be d i s p u t e d that the nature of the other i n h a b i t a n t s , with which each has to compete, i s ... g e n e r a l l y a f a r more important element of success (Darwin 1859, p. 400) . Bock (1972) has emphasized that b i o t i c i n t e r a c t i o n s provide a "constant m o d i f i c a t i o n of s e l e c t i o n f o r c e s " exerted by each s p e c i e s on other i n t e r a c t i n g s p e c i e s , and that t h i s stands i n sharp c o n t r a s t to the h i g h l y " s t a t i c s e l e c t i o n " exerted by the p h y s i c a l environment. These two types of s e l e c t i o n are not mutually e x c l u s i v e ; the e f f e c t of a b i o t i c s e l e c t i o n pressure may i n f l u e n c e the response to b i o t i c s e l e c t i o n pressure and v i c e v e r s a . P i g o t t (1982, p.402) draws a t t e n t i o n to the frequent o b s e r v a t i o n in v e g e t a t i o n s t u d i e s that p l a n t s "respond to changes in t h e i r n a t u r a l environment w i t h i n the context of the v e g e t a t i o n of which they are a p a r t " , i . e . that the r e a c t i o n of many i n d i v i d u a l s p e c i e s may r e s u l t from, and give r i s e t o , 5 responses of other s p e c i e s . Glesener & Tilman (1978) s t r e s s that under benign p h y s i c a l c o n d i t i o n s , the major s e l e c t i o n p r e s s u r e s r e l a t e d to environmental u n c e r t a i n t y w i l l be of b i o t i c o r i g i n and suggest that the changing genotypes of the organisms with which an i n d i v i d u a l i n t e r a c t s are the major source of t h i s b i o t i c u n c e r t a i n t y . C e n t r a l to much of community ecology i s the view that s p e c i e s c o e x i s t e n c e and community d i v e r s i t y r e f l e c t s b i o l o g i c a l accommodation among sp e c i e s (e.g. Sanders 1968). T h i s i s the process of e v o l u t i o n a r y adjustment of s p e c i e s to t h e i r b i o t i c environment. McNaughton & Wolf (1979) p o i n t out that no e m p i r i c a l attempt seems to have been made to estimate the extent of b i o l o g i c a l accommodation or i t s r o l e i n community o r g a n i z a t i o n . Antonovics (1978, p.246) w r i t e s "... ge n e t i c changes i n co m p e t i t i v e r e l a t i o n s have r a r e l y been s t u d i e d , e i t h e r i n e s t a b l i s h e d communities , durin g i n v a s i o n s or du r i n g e c o l o g i c a l s u c c e s s i o n . Such changes are very p e r t i n e n t to any d i s c u s s i o n of communities as coevolved systems, and i n need of i n v e s t i g a t i o n . . . . Even t h e . c o n f l i c t between, convergent a d a p t a t i o n to a common environment, and di v e r g e n t a d a p t a t i o n to other members of the community has never been e x p l i c i t l y i n v e s t i g a t e d " . To understand the nature of b i o l o g i c a l accommodation i s to understand the e v o l u t i o n a r y mechanisms f o r c o e x i s t e n c e . To approach t h i s problem i t i s necessary to recognize the most b a s i c components of the qu e s t i o n at hand, components which have g e n e r a l l y been n e g l e c t e d . I f c o m p e t i t i v e e x c l u s i o n i s to take p l a c e between two i n t e r a c t i n g s p e c i e s (besides the requirement 6 that resources be i n s u f f i c i e n t l y l i m i t e d supply) two e s s e n t i a l c o n d i t i o n s must e x i s t : 1) t h e i r resource requirements (or the number of niche dimensions i n which they i n t e r a c t ) must o v e r l a p beyond a c e r t a i n c r i t i c a l p o i n t , and 2) one of them must be a s u p e r i o r competitor f o r these common resource requirements. A comprehensive e v o l u t i o n a r y theory of c o e x i s t e n c e f o r p o t e n t i a l competitors must t h e r e f o r e encompass two d i s t i n c t components: 1) fundamental niche requirements and 2) r e l a t i v e c o m p e t i t i v e a b i l i t i e s . The f i r s t component has r e c e i v e d widespread t h e o r e t i c a l and e m p i r i c a l a t t e n t i o n (e.g. C o l l w e l l & Futuyma 1971, Pianka 1974, 1976, Schoener 1974, C o l l w e l l & Fuentes 1975, Whittaker & L e v i n 1975, Roughgarden 1976, Armstrong & McGehee 1976, 1980, Fenchel & C h r i s t i a n s e n 1977, van den Bergh & Braakhekke 1978, Newman 1982). The second component by comparison has r e c e i v e d almost none. Langford and B u e l l (1969) suggest that i n the study of the e v o l u t i o n of b i o t i c communities, i n v e s t i g a t i o n s of the processes i n v o l v e d i n the i n t e r a c t i o n among organisms should y i e l d r i c h rewards. E v o l u t i o n of b i o l o g i c a l accommodation i n i n t e r a c t i n g s p e c i e s w i l l a f f e c t community o r g a n i z a t i o n and d i r e c t the e v o l u t i o n of community c h a r a c t e r i s t i c s (Whittaker & Woodwell 1971, Whittaker 1975). Recent work has found evidence f o r d i f f e r e n t i a t i o n of s i n g l e p o p u l a t i o n s i n t o s p e c i a l i z e d ' b i o t i c ecotypes' produced i n response to d i f f e r i n g s e l e c t i o n pressures e x e r t e d by d i f f e r e n t neighbours (Watson 1969, L i n h a r t 1974, Turkington & Harper 1979c). The r e s u l t s of these recent developments have l e d some to advance a view f o r the p l a n t community which focuses on the i n d i v i d u a l genotype as the 7 p r i n c i p a l u n i t of community d i v e r s i t y (Antonovics 1976a, Harper 1977b, 1982). T h i s merges the o b j e c t i v e s of both p o p u l a t i o n and community ecology i n t o a common goal of understanding the consequences of s e l e c t i v e f o r c e s o p e r a t i n g on i n d i v i d u a l s . E f f o r t s are now needed to c h a r a c t e r i z e the p r e c i s e manner in which s e l e c t i o n produces b i o t i c s p e c i a l i z a t i o n of genotypes w i t h i n s i n g l e p o p u l a t i o n s and i t s r o l e i n the e v o l u t i o n of c o e x i s t e n c e and hence, of communities. The system under study in t h i s t h e s i s i s the pasture community. Pastures are g e n e r a l l y regarded as systems in which competition i s an important i n t e r a c t i o n (see e x t e n s i v e reviews by Donald (1963) and R i s s e r (1969)) and where ge n e t i c v a r i a t i o n in c o n s t i t u e n t species i s abundant and h i g h l y s u s c e p t i b l e to change in response to p h y s i c a l and b i o t i c f a c t o r s of the environment (see review by Snaydon (1978)). The aim of t h i s study i s to i n v e s t i g a t e the q u a l i t i e s of b i o t i c i n t e r a c t i o n s important in e v o l u t i o n a r y mechanisms of s p e c i a l i z a t i o n and accommodation amongst c o e x i s t i n g p l a n t s p e c i e s and how these c o n t r i b u t e to the s t r u c t u r e and e v o l u t i o n of the pasture community. The problem under i n v e s t i g a t i o n i s p r i m a r i l y c o n c e p t - o r i e n t e d rather than system-oriented. O b j e c t i v e s are not so much to understand more about pasture ecology as to study a system i n which a high i n t e n s i t y of neighbour i n t e r a c t i o n s i s p r e d i c t e d and i n which d i f f e r e n t stages of community development are a v a i l a b l e f o r i n v e s t i g a t i n g e v o l u t i o n a r y changes i n s p e c i e s i n response to t h e i r b i o t i c environment. Emphasis i s centered on the f o l l o w i n g q u e s t i o n s : 1) How do i n t e r s p e c i f i c c o m p e t i t i v e r e l a t i o n s change as a 8 community ages? 2) Is there g r e a t e r b i o t i c s p e c i a l i z a t i o n i n p o p u l a t i o n s from o l d e r communities? 3) Is s p e c i a l i z a t i o n r e c i p r o c a l between spe c i e s ? 4) Does b i o t i c s p e c i a l i z a t i o n occur at both the sp e c i e s and genotype l e v e l s ? 5) What i s the e v o l u t i o n a r y mechanism of b i o t i c s p e c i a l i z a t i o n ? 6) What r o l e does the e v o l u t i o n of b i o t i c s p e c i a l i z a t i o n , p e r m i t t i n g c o e x i s t e n c e , have in community e v o l u t i o n ? 9 TERMS AND CONCEPTS The terminology r e l a t e d to niche, competition and co e x i s t e n c e has been used i n many v a r i e d ways i n the l i t e r a t u r e . To a v o i d ambiguity i t i s necessary to d e f i n e the terms as they are used i n t h i s t h e s i s . The ch o i c e of meanings f o r terms and concepts i n ecology o f t e n r e f l e c t s the bi a s e s and o b j e c t i v e s of the p r a c t i t i o n e r . The same i s true here. I t i s proposed however that the semantic c o n f u s i o n surrounding the concepts of niche and competition i s r e l a t e d to the f a c t that an attempt to c h a r a c t e r i z e one i s o f t e n embedded i n a c h a r a c t e r i z a t i o n of the other, with the r e s u l t that n e i t h e r i s c l e a r l y understood. In what f o l l o w s , an attempt i s made to e l u c i d a t e o p e r a t i o n a l meanings f o r , and a d i s t i n c t i o n between, fundamental niche and co m p e t i t i v e a b i l i t y in p a r t i c u l a r r e f e r e n c e to p l a n t s . A) INTERACTION AND COEXISTENCE If two s p e c i e s in nature i n t e r a c t , t h i s i m p l i e s i n the most . ge n e r a l : sense that one' (or both) s pe c i e s i n some way enter (s) i n t o the sphere of i n f l u e n c e of the other at some time dur i n g t h e i r l i f e c y c l e s . Types of i n t e r a c t i o n recognized by e c o l o g i s t s i n c l u d e c o m p e t i t i o n , p r e d a t i o n , p a r i s i t i s m , mutualism, amensalism and commensalism. If a c e r t a i n i n t e r a c t i o n between the members of two p o p u l a t i o n s occurs r e p e a t e d l y (not n e c e s s a r i l y c o n t i n u a l l y but at l e a s t p e r i o d i c a l l y ) i n a given area, then the two sp e c i e s may be regarded as ' c o e x i s t i n g ' . In p l a n t s f o r example, a co m p e t i t i v e i n t e r a c t i o n between two sp e c i e s p o p u l a t i o n s may never occur 10 between v e g e t a t i v e i n d i v i d u a l s , but may r e p e a t e d l y occur between seeds f o r s i t e s s u i t a b l e f o r germination and e s t a b l i s h m e n t , or c o m petition may occur f o r p o l l i n a t o r s or d i s p e r s a l agents. If n e i t h e r s p e c i e s however has the p o t e n t i a l to c o n t i n u o u s l y enter i n t o the sphere of i n f l u e n c e of the other in any way, e i t h e r due to l i m i t a t i o n s imposed by d i s p e r s a l or by g e n e t i c a l l y determined s i t e requirements, then the two s p e c i es do not c o e x i s t . Coexistence then i s continuous i n t e r a c t i o n , and i n t e r a c t i o n i s an event i n which the a c t i v i t y of one organism a f f e c t s the a c t i v i t y of another. In t h i s t h e s i s , the term c o e x i s t w i l l be used in the above context, although Harper et a_l. (1961) use the term c o h a b i t i n a s i m i l a r sense, i . e . " i n d i v i d u a l s (or t h e i r products) ... come i n t o such p r o x i m i t y that a s t r u g g l e f o r e x i s t e n c e c o u l d reasonably be c o n s i d e r e d p o s s i b l e " . Werner (1979) suggests a d i s t i n c t i o n of terminology based on d i f f e r e n c e s i n temporal and/or s p a t i a l s c a l e : 'coexistence' i m p l i e s a s t a b l e e q u i l i b r i u m , i n c o r p o r a t i n g the n o t i o n of continuous i n t e r a c t i o n d e t a i l e d above; ' c o h a b i t a t i o n ' i m p l i e s a s h a r i n g of h a b i t a t or resources without i n t e r a c t i o n (or at l e a s t without continuous i n t e r a c t i o n ) ; and 'cooccurrence' or ' o c c u r r i n g together' i m p l i e s only a mutual presence in which no p a r t i c u l a r assumption i s made regarding any i n c i d e n c e of impact by one organism on another. 11 B) FUNDAMENTAL NICHE REQUIREMENTS The d i f f e r e n t meanings of the term 'niche' have been reviewed by s e v e r a l authors (MacArthur 1968, Vandermeer 1972, Whittaker et a l . 1973, H u r l b e r t 1981). E l t o n ' s (1927) ' a l l - i n c l u s i v e ' n o t i o n of the niche has found common usage (e.g. Krebs 197-9, R i c k l e f s 1979) and i s regarded as the s p e c i e s ' ' p o s i t i o n ' , ' r o l e ' or ' p r o f e s s i o n ' i n nature, i n c l u d i n g i t s c o m p e t i t i v e r e l a t i o n s h i p s with other s p e c i e s . Where the b a s i c niche concept has been used i n t e r p r e t a t i v e l y however, co m p e t i t i v e a b i l i t y has r a r e l y ever been regarded as p a r t of the niche of a s p e c i e s . The most c o n s i s t e n t usage of the term niche recognizes i t s e a r l i e s t c o n n o t a t i o n ( G r i n n e l l 1924, 1928) as the i d e a l i z e d d i s t r i b u t i o n of i n d i v i d u a l s i n the absence of i n t e r a c t i o n s with other s p e c i e s . G r i n n e l l ' s conception of the niche was " p r e - i n t e r a c t i v e - the p o t e n t i a l area w i t h i n which a s p e c i e s can l i v e as opposed to the area where one a c t u a l l y f i n d s i t " (Vandermeer 1972). T h i s g e n e r a l view i s r e f l e c t e d i n the l a t e r more s p e c i f i c conceptions of 'fundamental' versus ' r e a l i z e d ' niche (Hutchinson 1957), 'pre-competitive' versus 'post-competitive' niche (Levins 1968, C o l w e l l & Futuyma 1971), and ' p o t e n t i a l ' ' versus ' a c t u a l ' niche (MacMahon et a l . 1981). That c o m p e t i t i v e a b i l i t y i s not part of the c h a r a c t e r i z a t i o n of a s p e c i e s niche i s f u r t h e r evident from the c o m p e t i t i v e e x c l u s i o n p r i n c i p l e . V a r i o u s l y s t a t e d , the p r i n c i p l e e s s e n t i a l l y means that two species with very s i m i l a r niches cannot c o e x i s t - one w i l l out-compete the other. According to Gause's p r i n c i p l e , common niche r e s u l t s i n c o m p e t i t i v e e x c l u s i o n which n e c e s s a r i l y means d i f f e r e n t 1 2 c o m p e t i t i v e a b i l i t i e s ; t h i s c a t e g o r i c a l l y p r e c l u d e s c o m p e t i t i v e a b i l i t y from the d e f i n i t i o n of the n i c h e . E v i d e n t l y then, most e c o l o g i s t s have i m p l i c i t l y regarded c o m p e t i t i v e a b i l i t y as e x c l u s i v e of the p r i n c i p a l idea of the 'niche' i n s p i t e of c o n t r a d i c t o r y c laims ( f o l l o w i n g an E l t o n i a n view) that the niche r e p r e s e n t s a l l the r e l a t i o n s h i p s of the organism with i t s environment, which i n c l u d e s i t s c o m p e t i t o r s . T h i s r a i s e s the q u e s t i o n of the r e l a t i o n s h i p between fundamental niche and b i o t i c i n t e r a c t i o n in g e n e r a l . It i s proposed here that fundamental niche (or fundamental niche requirements) be c o n s i d e r e d as a l l p o s s i b l e s e t s of resources (e.g. energy, chemicals, c o n d i t i o n s of s i t e ) and resource s t a t e s (e.g. q u a n t i t y , q u a l i t y , i n t e n s i t y , r e n e w a b i l i t y , p e r i o d i c i t y , etc.) that meet minimal requirements f o r a s p e c i e s ( i n d i v i d u a l or p o p u l a t i o n ) to leave descendents. The a c t i v i t i e s of other s p e c i e s a f f e c t s the a v a i l a b i l i t y of r e q u i r e d resources to a r e f e r e n c e s p e c i e s through e f f e c t s on one or more of these resource s t a t e s . These e f f e c t s t r a n s l a t e i n t o b i o t i c i n t e r a c t i o n s - the a c t i v i t y of one organism i n f l u e n c e s the w e l l - b e i n g of another. Other organisms t h e r e f o r e a f f e c t the a v a i l a b i l i t y of niche requirements (and in turn a f f e c t f i t n e s s ) but i t i s c r u c i a l to recognize that they themselves are not part of the niche requirements (with the e x c e p t i o n of o b l i g a t e symbionts as i n l i c h e n s , or o b l i g a t e b e n e f a c t o r s such as p o l l i n a t o r s or i n d i v i d u a l s of the opposite gender in d i o e c i o u s s p e c i e s ) . T h i s p a r a l l e l s a recent view from H u r l b e r t (1981, p.177) : 13 As a consequence of i t s r e s o u r c e - u s i n g a c t i v i t i e s , an organism has many i n f l u e n c e s on the surrounding community or ecosystem. If the t o t a l i t y of these i n f l u e n c e s or impacts i s regarded as the ' f u n c t i o n a l r o l e ' of the organism, then ' r o l e ' i s not e q u i v a l e n t to niche, as many authors would have i t , but r a t h e r ' r o l e ' i s a consequence of the ni c h e . B i o t i c i n t e r a c t i o n s then are consequences of the niches of s p e c i e s . In p a r t i c u l a r , they are consequences of niche o v e r l a p between s p e c i e s . Organisms i n t e r a c t only i f they have at l e a s t some resource requirements i n common. Taking t h i s one step f u r t h e r , the c o e x i s t e n c e of organisms depends not only on t h e i r having some resouces requirements (niche) i n common, but a l s o on the impact that each has on the a v a i l a b i l i t y of resources r e q u i r e d by the other. T h i s two-component p e r s p e c t i v e f o r co e x i s t e n c e a p p l i e s to a l l b i o t i c i n t e r a c t i o n s and has been g e n e r a l l y recognized in predator/prey and h o s t / p a r a s i t e systems f o r example, but l a r g e l y n e g l e c t e d in systems of competition- because the n o t i o n of c o m p e t i t i v e a b i l i t y has been i n f u s e d i n t o the precept of ni c h e . F o l l o w i n g above arguments, i t i s proposed that fundamental niche requirements be regarded as a d i s t i n c t concept from c o m p e t i t i v e a b i l i t y and w i l l be d e f i n e d i n t h i s t h e s i s i n a pre- i n t e r a c t i v e sense most c l o s e l y resembling the hyper-volume view of Hutchinson (1957), i . e . a m u l t i d i m e n s i o n a l d e s c r i p t i o n of the whole range of resource requirements encompassing a l l environmental l i m i t a t i o n s of a s p e c i e s ( p o p u l a t i o n or i n d i v i d u a l ) w i t h i n which i t i s p o t e n t i a l l y capable of l e a v i n g descendents - "... p r i m a r i l y without r e f e r e n c e to competitors, but merely i n terms of requirements and t o l e r a n c e s ..." (Hutchinson 1978). 14 A h a b i t a t i n which a s p e c i e s i s capable of l i v i n g , r e p r e s e n t s an a c t u a l p r o p o r t i o n of the s p e c i e s ' fundamental niche that i s a v a i l a b l e for o c c u p a t i o n . In p r a c t i c e however, the presence of other s p e c i e s may deny occupation of c e r t a i n p o r t i o n s of a v a i l a b l e niche space, or they may expand the a v a i l a b l e niche space, and hence d e l i n e a t e s a ' p o s t - i n t e r a c t i v e ' r e a l i z e d niche w i t h i n the h a b i t a t . A c c o r d i n g l y , b i o t i c i n t e r a c t i o n i s a consequence of fundamental niche and r e a l i z e d niche i s a consequence of b i o t i c i n t e r a c t i o n . C) RELATIVE COMPETITIVE ABILITY The term 'competition' has come to have a v a r i e t y of meanings in ecology. In t h i s t h e s i s the s t r i c t e s t d e f i n i t i o n of the word w i l l be used f o l l o w i n g p r o p o s a l s in reviews by B i r c h (1957) and Milne (1961). Competition w i l l be d e f i n e d as an i n t e r a c t i o n between two (or more) organisms in which each reduces the a v a i l a b i l i t y of resource u n i t s to the other from a l i m i t e d supply on which they both make demands. T i n n i n (1972) p o i n t s to the v a r i a b l e and c o n f u s i n g r e l a t i o n s h i p . between 'competition' and ' i n t e r f e r e n c e ' i n the l i t e r a t u r e . A f u r t h e r v a r i a t i o n in terminology was i n t r o d u c e d by H a l l (1974a) who regards i n t e r f e r e n c e as any e f f e c t , d e t r i m e n t a l or b e n e f i c i a l , of one organism on another, the context in which the term ' i n t e r a c t i o n ' i s used p r e s e n t l y . In u n d e r l y i n g mechanisms to be c o n s i d e r e d l a t e r , the more t r a d i t i o n a l view that c o m p e t i t i o n can be d i v i d e d i n t o two types - ' e x p l o i t a t i o n ' and ' i n t e r f e r e n c e ' - (Park 1954, B i r c h 1957, M i l l e r 1967, 1969, Berendse 1981b) w i l l be adopted here. 1 5 An even more c o n f u s i n g term i n the l i t e r a t u r e than 'competition' i s 'competitive a b i l i t y ' . Competitive a b i l i t y alone i s m i s l e a d i n g as i t i s not an absolute c h a r a c t e r i s t i c of •an i n d i v i d u a l ; i t has meaning only i n a r e l a t i v e context to o t h e r s . Competitive a b i l i t y i s manifested only w i t h i n (and c h a r a c t e r i z e d e n t i r e l y by) the p r e v a i l i n g neighbourhood of i n t e r a c t i n g i n d i v i d u a l s making demands on the same l i m i t i n g r e s o u r c e ( s ) . Competitive a b i l i t y may be regarded as a combined measure o f : 1) the a b i l i t y of an i n d i v i d u a l to reduce the a v a i l a b i l i t y of l i m i t e d resources to another, and 2) the a b i l i t y to t o l e r a t e r e d u c t i o n i n resource a v a i l a b i l i t y by_ another. A t t r i b u t e s of a p l a n t which confer c o m p e t i t i v e a b i l i t y may in the f i r s t case be c o n s i d e r e d ' o f f e n s i v e ' c h a r a c t e r i s t i c s , and in the second case, 'defensive' c h a r a c t e r i s t i c s . O f f e n s i v e Character i st i c s C o n f e r r i n g Competitive A b i l i t y Numerous c h a r a c t e r i s t i c s of a p l a n t w i l l i n concert determine i t s o v e r a l l a b i l i t y to reduce the a v a i l a b i l i t y of resources to.another. O f f e n s i v e c h a r a c t e r i s t i c s important to co m p e t i t i v e a b i l i t y f a l l i n t o two c a t e g o r i e s , and d e f i n e two types of r e l a t i v e c o m p e t i t i v e a b i l i t y : 1) r e l a t i v e e x p l o i t a t i o n a b i l i t y - the r e l a t i v e e f f i c i e n c y of e x p l o i t a t i o n of re s o u r c e s , and 2) r e l a t i v e i n t e r f e r e n c e a b i l i t y - the r e l a t i v e l e v e l of i n t e r f e r e n c e with access to resources (Table 1). E x p l o i t a t i o n competition reduces the a v a i l a b i l i t y of l i m i t e d resources to another by reducing the q u a n t i t y of those resources by removal through uptake from the s i t e . T h i s r e q u i r e s a t t r i b u t e s r e l a t e d s o l e l y to e f f e c t i v e procurement of r e s o u r c e s . I n t e r f e r e n c e TABLE 1. P o s s i b l e c h a r a c t e r s which confer c o m p e t i t i v e a b i l i t y i n p l a n t s i n c o n t e x t s of a) r e l a t i v e e x p l o i t a t i o n a b i l i t y , and b) r e l a t i v e i n t e r f e r e n c e a b i l i t y . 17 a) EXPLOITATION COMPETITION 1. Rapid growth rate ( i . e . e f f e c t i v e photosynthetic metabolism and/or e f f e c t i v e or rapid uptake/storage of nutr ients and/or water). 2. T a l l plant he ight , long hypocotyls stems, pe t i o le s (Black I960) . 3. Large leaf area (Black I 960 ) . k. Leaf o r i en ta t i on for e f f ec t i ve capture of 1 i ght .(Mons i et_ aj_. 1973). 5. Deep and/or extensive root system. 6. Large embryonic capital (seed s ize) (Black 1958). 7. E f fec t i ve t iming (e.g. ear ly a r r i v a l at a s i t e , ea r l y germin- at ion and/or ear ly establishment) (e.g. Sagar 1959, Harper 1961). 8. A t t r a c t i v e f lowers, p o l l e n , seeds, or f r u i t s for e f fec t i ve p o l l i n - at ion and/or d i spersa l (Donald 1963, Levin & Anderson 1980, Davidson S Morton 1981) . 9. General adaptat ion, re s i s tence , or p l a s t i c i t y to 'adverse ' env i ron- mental condit ions other than those imposed by competitors (Daubenmire 1968). b) INTERFERENCE COMPETITION 1. Encouraging, she l te r ing or car ry - ing pests , disease organisms or predators which a f f e c t other species (Janzen 1966, Sandfaer 1968, 1970a, 1970b). 2. Encouraging/producing rhizosphere components or s o i l reactions un- favourable to other species (Trenbath 1976). 3. A l t e r i n g the a b i l i t y of the environment to provide s p e c i a l - ized t r i g ge r mechanisms (e.g. for the breakage of dormancy (Harper 1964)). k. Deposit ing a dense layer of l i t t e r on the ground surface (Grime 1973) . 5. Releasing substances which are di rect1y t ox i c to other species ( i . e . a l le lopathy) (Rice 197*0. 6. In ter fer ing with p o l l i n a t i o n (e.g. " po l l en a l l e lopa thy " ) (Kanchan 1980) . 18 c o m p e t i t i o n , i n c o n t r a s t , reduces the a c c e s s i b i l i t y of those resouces ( i . e . without resource removal). T h i s i n v o l v e s a t t r i b u t e s d i r e c t l y or i n d i r e c t l y r e l a t e d to p r e v e n t i n g a competitor from g e t t i n g a r e s o u r c e . Grime (1979, p.20) argues that an a b i l i t y to maximize dry matter p r o d u c t i o n i s a "general f e a t u r e " of c o m p e t i t i v e a b i l i t y i n p l a n t s . From Table 1 however, i t can be seen that a s u p e r i o r p l a n t competitor w i l l not n e c e s s a r i l y be the ' b i g g e s t ' , ' s t r o n g e s t ' , 'most a g g r e s s i v e ' , or 'highest y i e l d i n g ' (Harper 1965) and t h i s i s supported by evidence from c o m p e t i t i o n experiments with p l a n t s (de Wit 1 970) . It i s important to note that i n t e r f e r e n c e e f f e c t s may occur r e g a r d l e s s of niche o v e r l a p or resource l i m i t a t i o n , and as d e t r i m e n t a l e f f e c t s may only be i n c u r r e d by one s p e c i e s , such an i n t e r a c t i o n may be more s u i t a b l y r e f e r r e d to as 'amensalism'. (Harper (1961) has p r e f e r r e d to use the term i n t e r f e r e n c e to encompass both competition and amensalism, i . e . "... those hardships which are caused by the p r o x i m i t y of neighbours . . . " ) . I n t e r f e r e n c e phenomena however are u s u a l l y thought to be evolved responses to resource shortage imposed by c o m p e t i t i o n (e.g. G i l l 1974, Roughgarden 1979). Defensive C h a r a c t e r i s t i c s C o n f e r r i n g Competitive A b i l i t y In some s p e c i e s , higher r e l a t i v e c o m p e t i t i v e a b i l i t y may be c o n f e r r e d by a higher t o l e r a n c e d u r i n g p e r i o d s when resources are l i m i t e d by competition ( f o r example by d e l a y i n g or f o r g o i n g r e p r o d u c t i o n i n that year, v a r y i n g the number or s i z e of d i f f e r e n t organs, or by dying back above ground p a r t s but 19 a v o i d i n g t o t a l d eath). T h i s w i l l be p a r t i c u l a r l y important i n p l a n t s , where t o l e r a n c e a b i l i t y w i l l be a r e f l e c t i o n of such c h a r a c t e r i s t i c s as seed dormancy, l i f e forms with s u s t a i n i n g underground p a r t s (e.g. as in hemicryptophytes and geophytes), indeterminate growth and modular c o n s t r u c t i o n (Harper 1964, 1977b). Immunity to t o x i c a l l e l o p a t h i c substances produced by others would a l s o be a type of d e f e n s i v e c h a r a c t e r i s t i c c o n f e r r i n g c o m p e t i t i v e a b i l i t y . 20 PROGRAMME The s t u d i e s d e s c r i b e d i n t h i s t h e s i s are centered on a system of three c o e x i s t e n t stages of pasture community development. The approach to the problem under i n v e s t i g a t i o n i s t w o - f o l d : 1) t i m e - s e r i e s surveys of v e g e t a t i o n a l p a t t e r n s ; 2) experimental analyses of b i o t i c i n t e r a c t i o n s . The two approaches complement one another and each c o n t r i b u t e s i n f o r m a t i o n not a v a i l a b l e from the other. Percentage cover surveys of the three p a s t u r e s were c a r r i e d out p e r i o d i c a l l y over a 33-month p e r i o d to o b t a i n q u a n t i t a t i v e data f o r c h a r a c t e r i z i n g the developmental f e a t u r e s of pasture community e v o l u t i o n . The p r i n c i p a l aim was to o b t a i n an e m p i r i c a l b a s i s f o r r e f e r r i n g to the developmental r e l a t i o n s h i p of the three communities. Important assumptions however are recognized concerning the u n l i k e l i h o o d that the three pastures experienced i d e n t i c a l environmental c o n d i t i o n s (e.g. of c l i m a t e or grazing) at any given stage of development. .Quadrat sampling serves to r e v e a l gross v e g e t a t i o n a l p a t t e r n s , but i t i s not wholly s e n s i t i v e to the i n d i v i d u a l p l a n t ' s experience of i t s immediate neighbours. The s e s s i l e h a b i t of t e r r e s t r i a l p l a n t s and the h i g h sward d e n s i t y i n pastures means that a r e c o r d i n g of the number of above ground p h y s i c a l c o n t a c t s between i n d i v i d u a l s r e f l e c t s a parameter of f i n e - s c a l e i n t e r a c t i o n . If monitored over time t h i s procedure re p r e s e n t s a q u a n t i t a t i v e b a s i s f o r d i s c u s s i n g p a t t e r n s of l o c a l c o e x i s t e n c e at the l e v e l of i n d i v i d u a l experience, i . e . the " p l a n t ' s experience of community d i v e r s i t y " (Harper 1977b). 21 T h i s sampling method, known as 'contact sampling', i s used in the present study f o r t h i s purpose. I t was designed o r i g i n a l l y by Yarranton (1966) and has been used i n recent s t u d i e s i n pastures (Turkington et a l . 1 977, Aarssen et a_l. 1979, Turkington & Harper 1979a, 1979b, S a u l e i 1981). A l t e r n a t i v e s i n methodology and data a n a l y s i s as w e l l as i t s v e r s a t i l i t y and value as a method of v e g e t a t i o n sampling have r e c e n t l y been presented i n d e t a i l by de Jong e_t a l . ( i n p r e s s ) . A v a r i e t y of experimental methods were employed to i n v e s t i g a t e the b i o t i c i n t e r a c t i o n s o c c u r r i n g between c l o n e s of i n d i v i d u a l s which were in immediate p r o x i m i t y to one another as a c t u a l neighbours i n the f i e l d s and d i f f e r e n c e s in i n t e r a c t i o n s f o r the same s p e c i e s in pastures of d i f f e r e n t age. Use i s made of the mixture d i a l l e l (e.g Trenbath 1978) and replacement s e r i e s (de Wit 1960) d e s i g n s . A f u r t h e r method of r e c i p r o c a l phytometer t r a n s p l a n t i n g i s d e v i s e d and used to i n v e s t i g a t e r e c i p r o c a l b i o t i c s p e c i a l i z a t i o n i n two s p e c i e s i n response to d i f f e r e n t neighbouring genotypes of the same sp e c i e s c o l l e c t e d from d i f f e r e n t neighbourhoods i n the o l d e s t pasture ( i . e . b i o t i c s p e c i a l i z a t i o n at the genotype l e v e l ) . Where a p p r o p r i a t e , r e s u l t s are i n t e r p r e t e d by p l a c i n g them i n t o a t h e o r e t i c a l and e v o l u t i o n a r y c o n t e x t . T h i s i s pursued at l e n g t h i n the f i n a l chapter which e x p l o r e s the i m p l i c a t i o n s of n a t u r a l s e l e c t i o n o p e r a t i n g in systems of c o m p e t i t i o n . A general e v o l u t i o n a r y theory of c o e x i s t e n c e i s proposed and d i s c u s s e d i n r e l a t i o n to other t h e o r i e s of c o e x i s t e n c e i n contexts of c o m p e t i t i o n . 22 CHAPTER 2 THE STUDY SITE: VARIABLES AND PATTERNS 23 INTRODUCTION The study s i t e c o n s i s t s of three pastures l o c a t e d on the d a i r y farm owned by W i l l i a m and Mary Chard, 25704 Fr a s e r Highway, Aldergrove (SW 1/4 Sec. 25, Twp. 10) i n the F r a s e r V a l l e y of B r i t i s h Columbia (49°03'45"N L a t . ; 122°30' 45"W Long.). The farm i s s i t u a t e d i n the C o a s t a l D o u g l a s - f i r b i o g e o c l i m a t i c zone of B.C. ( R r a j i n a 1965), but has been managed as g r a z i n g lan d s i n c e the turn of the c e n t u r y . The three pastures were l a s t ploughed and seeded i n 1939, 1958, and 1977 and h e r e i n a f t e r the f i e l d s w i l l be named a f t e r these dates or w i l l be r e f e r r e d to as the 40, 21 and 2 y e a r - o l d pastures ( r e s p e c t i v e l y ) i n d i c a t i n g the ages of the f i e l d s when cl o n e s were c o l l e c t e d f o r competition experiments. The f i e l d s were a l s o used as pastures p r i o r to the above most recent p l a n t i n g dates. D e t a i l s are summarized in Table 2. TABLE 2. D e t a i l s on the three study f i e l d s . Approx. date of c l e a r i n g (from Age when scrub/brush) Date of reseach commenced and f i r s t l a s t ploughing (years s i n c e ploughing and seeding . l a s t ploughed) S i z e (ha) 1 942 1 977 2 1 .04 1930 1958 21 1 .44 1900 1939 40 0.99 The pastures have always been e i t h e r grazed by c a t t l e or o c c a s i o n a l l y cut f o r hay. G r a z i n g i n a l l three f i e l d s i n v o l v e s on average, 20 to 30 cows present i n t e r m i t t e n t l y from May u n t i l 24 November. Weather c o n d i t i o n s r a r e l y permit g r a z i n g between December and A p r i l . None of the f i e l d s have been reseeded s i n c e the s t a t e d dates, nor have they ever r e c e i v e d any chemical treatment ( f e r t i l i z e r s or p e s t i c i d e s ) . Barnyard manure i s m e c h a n i c a l l y spread p e r i o d i c a l l y . The 1939 and 1977 pastures are g e n e r a l l y f l a t with a s l i g h t slope i n the 1977 f i e l d . The 1958 f i e l d i s g e n t l y r o l l i n g . The e l e v a t i o n of the s i t e v a r i e s from 110 to 122 m above sea l e v e l . Contour maps of the f i e l d s are shown i n F i g u r e 1. The pastures are contiguous ( F i g . 1) but separated from one another by rows of t r e e s . The e n t i r e s i t e i s enclosed by wire f e n c i n g and bordered by drainage d i t c h e s and laneways, and i n some p l a c e s , t r e e s . THE GEOLOGY The general area i s u n d e r l a i n by t r u n c a t e d s t r a t a of Eocene sediments upon which l i e a parent m a t e r i a l of g l a c i o m a r i n e d e p o s i t s of l a t e s t P l e i s t o c e n e age (11,000 - 12,000 years B.P.). The study s i t e s p e c i f i c a l l y has moderately to f i n e - t e x t u r e d c l a y loam, i s g e n e r a l l y non-stbney. and poor to moderately w e l l d r a i n e d . The s o i l i s L u v i s o l i c Humo-Ferric Podzol and O r t h i c Humic G l e y s o l (Canada S o i l Survey Committee 1978). Average top s o i l depth (down to the top of the 'C horizon) i s 80 - 100.cm. 25 FIGURE 1. Contour maps of the study f i e l d s . E l e v a t i o n s are given i n meters above sea l e v e l . P o i n t s represent the l o c a t i o n s of 60 quadrats i n each f i e l d used i n the v e g e t a t i o n surveys. I n s e t s show the p o s i t i o n a l r e l a t i o n s h i p of the three p a s t u r e s .   28 29 THE CLIMATE Most c l i m a t i c data were recorded at Aldergrove, 2.4 km east of the study s i t e . The study area f e a t u r e s a r e l a t i v e l y m i l d winter with heavy p r e c i p i t a t i o n , mostly i n the form of r a i n f a l l , averaging 164.3 cm per annum (1953-1970). In winter the area i s u s u a l l y not covered by snow f o r p e r i o d s l a s t i n g longer than 1 or 2 weeks. The mean monthly d i s t r i b u t i o n s of r a i n f a l l , s n o w f a l l and t o t a l p r e c i p i t a t i o n are shown i n Table 3. TABLE 3. The monthly d i s t r i b u t i o n s of a) r a i n f a l l (cm), b) s n o w f a l l (cm) and c) t o t a l p r e c i p i t a t i o n (cm) (means f o r 1953- 1970, A l d e r g r o v e ) . Jan Feb Mar Apr May June J u l y Aug Sept Oct Nov Dec a) 18.7 16.5 14.6 10.9 7.2 6.6 4.3 5.2 9.4 18.5 20.7 22.0 b) 41.9 12.4 10.4 0.5 0.3 0.0 0.0 0.0 0.0 0.0 5.1 25.4 c) 22.9 17.8 15.6 10.9 7.2 6.6 4.3 5.2 9.4 18.5 21.2 24.5 The area i s exposed to predominantly north and n o r t h - e a s t e r l y winds i n the f a l l and winter months. Winds are mostly from the south and south-west i n s p r i n g and summer. For the years 1953 to 1970 the maximum summer a i r temperature recorded was 36?C while the minimum winter temperature was -20 0 C. Monthly d i s t r i b u t i o n s of c l i m a t i c v a r i a b l e s r e l a t e d to temperature and hours of sunshine are shown i n Table 4. 30 TABLE 4. The monthly d i s t r i b u t i o n s o f : a) mean d a i l y temperature (°C), b) mean d a i l y maximum temperature (°C), c) mean d a i l y minimum temperature (°C); d) number of days with f r o s t ( d a i l y minimum below 0 °C) (means f o r 1953-1970, A l d e r g r o v e ) ; e) number of hours of sunshine (means f o r 1970- 1980, P a c i f i c Regional Atmospheric Environment S e r v i c e ) . Jan Feb Mar Apr May June J u l y Auq Sept Oct Nov Dec a) 0.8 3.3 4.9 7.6 11.5 14.2 16.1 16.0 13.6 9.3 4.9 2.5 b) 4. 1 7.6 9.8 13.3 17.8 20.2 23.2 22.9 20.3 14.6 8.8 5.7 c) -2.4 -0.9 0.0 1 .9 5.2 8.2 8.9 9.1 6.9 4. 1 1 . 1 -0.7 d) 21 1 5 1 7 9 3 0 0 0 1 5 1 3 1 7 e) 68 77 1 1 2 1 64 209 217 291 245 1 74 1 37 72 54 THE SPECIES A t o t a l of 28 n a t u r a l i z e d herbaceous s p e c i e s are present i n the 3 pastures, 11 of which are grasses (Table 5). A l l three f i e l d s were l a s t sown with a seed mixture comprising 5-10% T r i f o l i u m repens , 15-20% D a c t y l i s glomerata and 70-80% 'High- land' forage mixture (Table. 6 ) . . The same s p e c i e s were present in t h i s mixture at a l l three seeding times (Richardson Seed Co., pers. comm.) but in f o r m a t i o n on t h e i r r e l a t i v e p r o p o r t i o n s i s a v a i l a b l e only f o r the mixture used i n p l a n t i n g the 1977 pasture (Table 6). 31 TABLE 5. A s p e c i e s l i s t f o r GRASSES Agropyron repens (L.) Beauv. A q r o s t i s a l b a L. Anthoxanthum odoratum L. D a c t y l i s glomerata L. Festuca rubra L. Holcus l a n a t u s L. Lolium mult i florum Lam. Lolium perenne L. Phleum pratense L. Poa compressa L. Poa t r i v i a l i s L. the study s i t e . NON-GRASSES A c h i l l e a m i l l e f o l i u m L. Carex sp. Cerastium vulqatum L. Cirsiurn arvense (L.) Scop. Hypochoeris r a d i c a t a L. Juncus sp. Medicaqo l u p u l i n a L. Plantaqo l a n c e o l a t a L. Plantaqo major L. Ranunculus a c r i s L. Rumex a c e t o s e l l a L. Rumex c r i s p u s L. Rumex o b t u s i f o l i u s L. S t e l l a r i a media (L.) V i l l . Taraxacum o f f i c i n a l e Weber T r i f o l i u m repens L. T r i f o l i u m pratense L. A) PATTERNS IN THE VEGETATION Each of the three pastures was surveyed f o r percentage cover of s p e c i e s on 9 o c c a s s i o n s over a 33-month p e r i o d . Three s p r i n g (March), three summer (June) and three f a l l (September) surveys were c a r r i e d out. The surveys were conducted i n order to t e s t two hypotheses: 1) Species composition i s the l e a s t s i m i l a r between the youngest and o l d e s t p a s t u r e s ; 2) Species 32 composition i s the most v a r i a b l e throughout the 33-month p e r i o d i n the youngest community and the l e a s t v a r i a b l e in the o l d e s t community. TABLE 6. Species composition of the 'High-land' forage mixture used i n sowing the three p a s t u r e s . P r o p o r t i o n s shown are f o r the 1977 mixture only. Lolium perenne was a t e t r a p l o i d v a r i e t y in the 1977 mixture. D a c t y l i s glomerata 45% T r i f o l i u m pratense 20 Lolium perenne 15 Lolium m u l t i f l o r u m 10 Phleum pratense 5 T r i f o l i u m repens 2 Ladino Clover ( T^ repens ) 3 (1977 only) Methods Each survey. was based on data c o l l e c t e d from 25 s y s t e m a t i c a l l y p o s i t i o n e d p o i n t s (5 rows of 5) w i t h i n each of 60 s y s t e m a t i c a l l y arranged 'permanent' quadrats, each 0.5 m x 0.5 m, g i v i n g a t o t a l of 1500 p o i n t s per f i e l d . R e l a t i v e cover r e s u l t s f o r an i n i t i a l survey of 2000 p o i n t s ( i n 80 quadrats) per f i e l d were not s u b s t a n t i a l l y changed a f t e r randomly d i s c a r d i n g up to 500 p o i n t s from the data. To minimize f i e l d e f f o r t , a l l subsequent surveys were t h e r e f o r e based on 1500 p o i n t s per f i e l d . Each quadrat was approximately 10 m from each neighbouring quadrat and a border of at l e a s t 10 m was avoided 33 as a b u f f e r zone around the perimeter of each pasture ( F i g . 1). Estimates of percentage cover were obtained by r e c o r d i n g a l l d i f f e r e n t s p e c i e s encountered in a downward ( v e r t i c a l ) p r o j e c t i o n at each p o i n t and expressing these i n terms of percentage cover f o r every 25 p o i n t s examined. The t o t a l percentage of ground covered by the s p e c i e s i n any one quadrat w i l l exceed 100% because the leaves of more than one s p e c i e s may cover the same p o i n t on the ground. Due to d i f f i c u l t y of quick d i f f e r e n t i a t i o n between Lolium m u l t i f l o r u m and Lolium perenne and because they are known to h y b r i d i z e r e a d i l y in nature (Hubbard 1968), the two s p e c i e s were lumped together and recorded as one s p e c i e s . Upon c a r e f u l i n s p e c t i o n , specimens of Lolium m u l t i florum were only i d e n t i f i e d i n the 1977 f i e l d ; a l l occurrences of Lolium spp. were t h e r e f o r e recorded as Lolium perenne . To analyze the percentage cover data, a r e c i p r o c a l averaging, e i g e n v e c t o r method of o r d i n a t i o n was employed using a program w r i t t e n by Dr. Gary B r a d f i e l d developed a f t e r the a l g o r i t h m o u t l i n e d i n O r l o c i (1978). R e s u l t s The mean t o t a l percentage cover over the study p e r i o d f o r the 1977, 1958 and 1939 pastures was 159%, 186% and 212% r e s p e c t i v e l y . The r e l a t i v e r e p r e s e n t a t i o n s of the most abundant s p e c i e s i n the 3 f i e l d s are shown for each survey i n F i g u r e 2, and as means over the study p e r i o d i n F i g u r e 3. Some trends are n o t a b l e . With i n c r e a s i n g f i e l d age, D a c t y l i s glomerata shows a g e n e r a l d e c l i n e i n percentage cover while Holcus l a n a t u s , Poa compressa and T r i f o l i u m repens show an i n c r e a s e i n cover. 34 FIGURE 2. T o t a l percentage cover of the 14 most abundant s p e c i e s present i n the three pasture communities over the 33- month sampling p e r i o d ; 1977 f i e l d ; • 1958 f i e l d ; 1939 f i e l d . 35 PERCENT COVER a) Agropyron repens i 1 1 1 1 1 1 r i 1 1 1 1 1 1 1 i June Sept Mar June Sept Mar June Sept Mar 1979 1980 1981 1982 36 PERCENT COVER 20 15 i 10 A 5 i d) Festuca rubra e) Holcus l a n a t u s 40 I 30 20 A 10 T r i 1 r T 1 r f ) Lolium perenne 40 4 30 H 20 10 H - i 1 1 r Har June Sept Mar —I 1— June Sept i 1 r~ Mar June Sept 1979 1980 1981 1982 37 1979 1980 198l 1982 38 PERCENT COVER i ) Plantago l a n c e o l a t a 1 1 —I 1 1 1 1 1 1 —I r— June Sept 1979 Mar 1 - June 1980 'Mar 1982 —J— Sept —r~ Mar —1 V June Sept 1981 39 PERCENT COVER 10 A m) T r i f o l i u m pratense T r T r T r 70 60 A 50 J 40 30 20 -I 10 A n) T r i f o l i u m repens T r i r 10 n June Sept 1979 o) Other s p e c i e s Mar June Sept 1980 1 1 r Mar June Sept 1981 1 Mar 1982 40 FIGURE 3. The 10 most abundant s p e c i e s present i n each study f i e l d based on mean percentage cover over the study p e r i o d . Standard e r r o r s are i n d i c a t e d by v e r t i c a l bars. The 5 sp e c i e s c o l l e c t e d and clo n e d f o r co m p e t i t i o n experiments are shown by c r o s s - h a t c h i n g and s t i p p l i n g . D i f f e r e n c e s i n percentage cover of these 5 s p e c i e s depending on f i e l d age are s i g n i f i c a n t at P<0.05 from an a n a l y s i s of v a r i a n c e using a MIDAS program (Fox & Guire 1976). A n a l y s i s was based on log-transformed data wherever an F - t e s t i n d i c a t e d unequal v a r i a n c e s i n the raw data. Ar - Agropyron repens Dg - D a c t y l i s glomerata Fr - Festuca rubra HI - Holcus l a n a t u s Lp - Lolium perenne Pp - Phleum pratense P i - Plantago l a n c e o l a t a Pc - Poa compressa Ra - Ranunculus a c r i s , To - Taraxacum o f f i c i n a l e Tp - T r i f o l i u m pratense Tr - T r i f o l i u m repens PERCENT COVER 40 A 1977 PASTURE Dg Pc Lp Tr To Ar HI Ra PI Tp 50 40 - 30 - 20 - 10 0 1958 PASTURE Pc Tr Dg Fr HI To Ar Lp Pp Ra 1939 PASTURE Pc Tr HI Lp To Ra Dg Pp Ar Fr 42 Festuca rubra has the hi g h e s t cover i n the middle-aged (1958) pasture compared to i t s cover i n the other p a s t u r e s . These o v e r a l l v e g e t a t i o n a l d i f f e r e n c e s among the 3 pa s t u r e s are summarized in an o r d i n a t i o n of the 27 surveys (3 pastures x 9 sampling events) based on t o t a l percentage cover f o r each s p e c i e s over the 60 quadrats i n each f i e l d ( F i g . 4). The three pastures are r e a d i l y d i s t i n g u i s h e d along the a x i s 1 plane which accounts f o r 42% of the v a r i a t i o n i n the data. The r e s u l t s show that the q u a n t i t y of v a r i a t i o n accounted f o r by the f i r s t 3 axes of the o r d i n a t i o n g e n e r a l l y d e c l i n e s with i n c r e a s i n g f i e l d age. Ax i s 2 accounts f o r a p p r e c i a b l e v a r i a t i o n i n a l l three p a s t u r e s . For axes 1 and 3 however, the q u a n t i t y of v a r i a t i o n accounted for decreases with i n c r e a s i n g pasture age; an F - t e s t f o r the e q u a l i t y of v a r i a n c e s of a x i s scores f o r the three pastures was s i g n i f i c a n t at P<0.05 f o r both axes 1 and 3. Time-s e r i e s course l i n e s showing s u c c e s s i v e sampling events f o l l o w the same d i r e c t i o n i n a l l three f i e l d s along the a x i s 2 plane. The course l i n e f o r the 1977 f i e l d i s the most d i r e c t i o n a l proceeding towards the p o s i t i o n i n g of the 1958 pasture surveys on a x i s 1. In c o n t r a s t , the course l i n e f o r the 1939 pasture on the a x i s 3 versus a x i s 1 p l o t appears as an ' o s c i l l a t i o n around a mean'. B) COLLECTION AND PROPAGATION OF CLONES In order to i n v e s t i g a t e f i n e - s c a l e c o m p e t i t i v e r e l a t i o n s , s e v e r a l p a i r s of n a t u r a l l y neighbouring 'genets' (or clo n e s ) (growing i n p h y s i c a l c o n t a c t above ground i n the f i e l d ) were c o l l e c t e d from l o c a t i o n s i n a l l 3 pastures where there was the 43 FIGURE 4. O r d i n a t i o n r e s u l t s of the 27 surveys. a) a x i s 2 vs. a x i s 1. b) a x i s 3 vs. a x i s 1. T i m e - s e r i e s course l i n e s are drawn f o r each pasture showing the p r o g r e s s i o n from the f i r s t ( c i r c l e d p o i n t ) to the n i n t h sampling event; 1 977 f i e l d ; . 1958 f i e l d ; 1939 f i e l d . 44 1939 1977 / \ \ \ 1958 * \ / » \ 6 \ + AXIS 1 ( 4 1 . 7% <»f t o t d l v a r i a n c e ) AXIS 1 45 g r e a t e s t o v e r l a p in t h e i r r e s p e c t i v e percentage cover f r e q u e n c i e s i n the f i r s t survey (June, 1979). Clones were c o l l e c t e d i n a l l p o s s i b l e p a i r s f o r 5 of the most abundant s p e c i e s common to a l l three p a s t u r e s ; they were D a c t y l i s glomerata, Holcus lana t u s , Lolium perenne , Poa compressa and T r i f o l i u m repens ( F i g . 3 ) . Each genet was given a 'type' name X/Y, where X i s i t s s p e c i e s name and Y i s the s p e c i e s name of i t s n a t u r a l neighbour (and X does not equal Y) Hence, X/Y and Y/X would be a n a t u r a l neighbouring genet p a i r . The p a i r s were c o l l e c t e d as two whole p l a n t s ( i n c l u d i n g root m a t e r i a l ) and each genet type was propagated s e p a r a t e l y under glasshouse c o n d i t i o n s by p e r i o d i c s e p a r a t i o n of t i l l e r s or c u t t i n g of s t o l o n p i e c e s f o l l o w e d by r e p l a n t i n g as 'ramets' ( p h y s i o l o g i c a l l y independent i n d i v i d u a l s ) of the o r i g i n a l c l o n e . Each such p o p u l a t i o n of ramets provided a stock supply of cloned i n d i v i d u a l s of the genet type. In experimental setups, a 'ramet' was designated as one 8-10 cm t i l l e r from a grass clone with a l l root m a t e r i a l removed, or an approximately 2.5 cm s e c t i o n of white c l o v e r s t o l o n with one t r i f o l i a t e l e a f and an a s s o c i a t e d a x i l l a r y bud. SOIL SAMPLING AND ANALYSES S o i l samples from a l l three f i e l d s were analyzed f o r the f o l l o w i n g v a r i a b l e s : pH ( i n water), t o t a l phosphorus, t o t a l n i t r o g e n , t o t a l carbon, t o t a l c a t i o n exchange c a p a c i t y , and exchangable calcium, magnesium, potassium and sodium. 46 Methods A r e c i p r o c a l averaging o r d i n a t i o n of quadrats (based on sp e c i e s cover) from the March (1981 ) survey (the most recent survey before s o i l sampling) was c a r r i e d out f o r a l l three f i e l d s . From the a x i s 2 vs a x i s 1 p l o t of these o r d i n a t i o n s , 20 stands from each of the three f i e l d s were s u b j e c t i v e l y chosen to represent the range of v a r i a t i o n present w i t h i n each f i e l d . These 20 stands were then s u b j e c t e d to r e - o r d i n a t i o n . The o r d i n a t i o n s so produced formed the b a s i s of the environmental o r d i n a t i o n s shown i n F i g u r e 9 used to t e s t the hypothesis that v e g e t a t i o n d i f f e r e n c e s w i t h i n a given f i e l d can be accounted f o r by u n d e r l y i n g edaphic v a r i a t i o n . At each of the 20 s e l e c t e d s i t e s i n each f i e l d , s o i l samples were taken in May 1981 by the composite method (e.g. Chapman et a_l . 1940) as f o l l o w s : 12 s o i l c o r e s , each 15 cm deep by 2 cm diameter, were e x t r a c t e d from around the o u t s i d e perimeter of the p o s i t i o n e d quadrat so as not to d i s t u r b the i n t e r i o r of the quadrat. These 12 independent samples were amalgamated i n t o the composite sample which gave a mean a n a l y t i c a l value r e p r e s e n t a t i v e of the e n t i r e s o i l sampling volume at the quadrat s i t e . Each composite sample was a i r d r i e d f o r 14 days, s i f t e d through a 2 mm si e v e and thoroughly mixed i n p r e p a r a t i o n f o r s o i l a n a l y s e s . A l l a n a l y s e s were c a r r i e d out on the 2 mm f r a c t i o n by procedures o u t l i n e d i n L a v k u l i c h (1978). Phosphorus was e x t r a c t e d u s i n g the ammonium f l u o r i d e method. A c o l o u r i m e t r i c procedure by auto-analyzer was used f o r t o t a l n i t r o g e n d e t e r m i n a t i o n . T o t a l carbon a n a l y s i s was obtained using a 'Leco 47 FIGURE 5. Mean values f o r chemical analyses of s o i l from 20 s e l e c t e d stands from each pasture (4, 23 and 42 years o l d ) . T o t a l c a t i o n exchange c a p a c i t y (CEC) and a l l exchangeable c a t i o n s are expressed as meq per 100 g s o i l . 48 x x a c •H 05 O X Oh a a • J 2; < U3 H O O O H OS H Vi H 2; 2 O CO < u I—I < H O 5.60H 5.50 5.40' 28.0 26.0 24.0 22.0 20.0 J8v0 0.43 0.42 0.41 0.40 5.80 5.60 5.40 5.20 5.00 4.80- 4.60- 38.D-, 36, OH O w 34.0 32.0 30.0 4.6 n 4.4 4.2 4.0 3.8 3.6 3.4 1.10 1.00 + + to + + S 0.90 0.80 j 0.25 -, %' 0.20 0.15 0.08 0.07 + CO 55 0.06 0.05 23 42 1 •4- YEARS SINCE PLANTING 49 FIGURE 6. D i s t r i b u t i o n a l p a t t e r n s of the 9 s o i l v a r i a b l e s on the a x i s 2 versus a x i s 1 plane of the stand o r d i n a t i o n of 20 s e l e c t e d quadrats from each p a s t u r e . Values f o r exchangable c a t i o n s are i n meq per 100 g of s o i l . 50 P H •5.25 • 5.20 1977 •5.65 * 5.60 • 5.'l5 5.65 *5.55 •5.70 *5.50 •5.60 •5.60 , 5.20 *5.50 •5.50 1.50 •5.10 5*.50 • 5.10 5.20 P • 21.1 • 12.1 1977 •̂ 27.1 . 15.1 •19.2 •27.1 • 15.1 •16.1 •19.2 •26.2 •20.8 , • 11.3 • 11.5 16.9 •18,9 21.6 -21.7 •21.7 13.5. •5.50 p H •5.30 1958 •5.50 • 5,30 5.60 • 5>75 5.50« • 5.60 • 5.65 515 ^ •5.15 ' 5 w , , "5.55 5.80 5.60 • 5.80 • 5.60 • 5,60 • 5,70 •11.5 • 12.2 • 12,0 > 21.6 P 1958 •15.1 23.E •  2y- 13.2 . . * 23.9 ig.e 16.1 25.8 • 25,5 26,1 51 '0.416 N * 0.463 1977 •0.425 •0.259 •0.431 •0.278 •0.366 • 0.278 • 0.469 .0.525 • 0.466 0*469 *0.53 0.463 0.522 •0.425 • 0.431 • 0.447 0.328 C • 5,44 • 5.77 1977 •5.92 • 3.39 •5,07 •4,43 •5.11 •3.94 •6.86 •6.09 •5.39 , •6.60 • 4.99 6.71 ,•5.80 5.97 •E.16 . e „ * £,91 E.75 N 1958 • 0.372 0,481 0.391 • 0.422 0.491- 0.372 0 . 4 3 1 . ° ' ^ * \ . •0.4280.419 0.366 0.338 • 0.384 •0.366 • 6.42 •0.494 N •0.347 •0.388 1939 •0.375 • 0.369 •0.369 •0.469 0.403 .0.400 • 0.403 0.350 • 0.341 0.413 •0.369 •0.366 • 0.422 ^ • 5.58 6,44 • 5.65 c 1958 7,66 • 5.3D 6,12 - 4.79 7.08 - . . * 6-39 6,03 5.03 4.56 • 5.69 5.95 52 CEC •37.6 •35.8 1977 •37.6 •31.3 -3M.0 •31.0 •36.7 • 31.0 * m.t • 37.6 • 32.2 , •10.3 •31.9 39.1 ,• 12.1 11.2 • 5 9 ' " -10.3 •15.6 12.; •3.0 Ca + + 1977 •5.7 • 3.6 •3.9 •1.2 •3.1 *3.6 •6.1 •59 * 5 - 9 /1.2 1.7 -5.9 ? • 5.1 32.2 • 30.1 • 36.7 • 30.1 CEC 1958 > 31.3 • 32.2 35.8 • 37.6 11.2 • 37,61 •30,1 . a i 9 5.8 . . * 35.7 ia.2 10.3 33.1 • 31.0 . 36,7 • 3.6 Ca + + • 2.1 1958 • 3.1 •3.6 1.9 • 3,6 6,1 • • 2.5 • 1.6 5.1 6'1 1.9 . . "6.9 5.1 1.1 3.2 • 1.2 •1,0 • 3.9 • 3.5 •32.2 CEC 1939 • 26.9 •31.3 •30.1 • 36.7 •31.3 •26.9 3Q.1 26.0 •26.0 32.2 • 21.2 •29.5 •26.0 •32:2 • 38.5 *33.1 y, Q •32.2 •27.8 •3.7 Ca + + 1939 •1.0 •3.9 •2.9 •1.2 •1.9 •1.0 3.7 3.6 •3.6 3.7 •1.7 •2.5 •2.9 •1.7 •1.1 •3.7 •3.6 ' M •3.0 53 M g + + 1977 •0.49 •0.43 •1.26 .0.95 •0.62 •0.84 •0.62 • 0.82 • 0.97 • 1.03 • 0.97 . •1.11 •1.01 0.99 ,•0.70 0.97 *0.82 0.88 • 0 19 K + •0.17 1977 • 0.19 • 0.17 •0.2C 0.13 •0.20 • 0.17 • 0.17 • 0.06 • 0.19 •0.26 •0.13 •0.83 / 0.06 0*.26 • 0.19 • o.oe • 0,13 0.06 • 0.64 • 0.37 1958 •0.88 •0,72 2.18- l ; * 0,70 • • 1.42 1.4E • 1.85 0.86 3,84 •- ~ • . . * I .* 1.50 0.80 0.66 • 0.99 „ „ •2.06 • 1.07 • 0.51 • 0.16 • 0,19 K + 1958 > 0,16 • 0.06 0.13 - H.19 • 0.20 * ° ' 1 9 -CIO °- ' X . , * 0.1E 0.17 0.16 -0,17 0.37 •0,70 M g + + 1939 •1.07 •0.80 •1.15 • 0,41 • l . U 0,86 > 0.86 • 1.03 a.si 0.76 •0.64 •1.30 •0.58 •1.69 •0.41 .1.07 • 0.76 54 •0.O5 Na+ •0.01 1977 •0.08 •0.11 •0.05 •0.10 •0.03 • 0.08 • 0.09 • 0.17 • 0.01 , •0.08 •0.13 0.09 /0.05 • 0.09 * 0 - r o • 0.10 • 0.01 o.qp • 0.07 Na+ • 0.03 1958 •0.07 • 0.05 0.08 • P.-09 0.15 • • 0.05 • 0.09 •0.05 . „ • 0.05 0.11 0 - « * ° ^ . ' 0 . 1 2 o.lO 0.08 0.01 08 • 0.23 •0.08 N a .o.io 1939 •o.oi • 0.08 •0.01 OJ33 .n(K * ° - Q 2  u , l c • 0.01 O.Oi •0.01 •0.03 • 0.02 •OvOl • 0.08 •0.09 „ m-0.15 •0,03 •0.07 55 a n a l y z e r ' . The ammonium ace t a t e method was used for exchangable c a t i o n s and t o t a l c a t i o n exchange c a p a c i t y (CEC). R e s u l t s The mean r e s u l t s are summarized in F i g u r e 5. A t e s t of s i g n i f i c a n t d i f f e r e n c e s i n mean values f o r s o i l v a r i a b l e s between f i e l d s was not p o s s i b l e as s o i l samples were not c o l l e c t e d randomly. F i g u r e 6 i l l u s t r a t e s the d i s t r i b u t i o n a l p a t t e r n s of the measured s o i l v a r i a b l e s on the stand o r d i n a t i o n s f o r each f i e l d . Each diagram i s based on the a x i s 2 versus a x i s 1 plane of the o r d i n a t i o n of the 20 s e l e c t e d quadrats mentioned p r e v i o u s l y . C o r r e l a t i o n c o e f f i c i e n t s f o r the d i f f e r e n t s o i l v a r i a b l e s with the percentage cover of 10 s p e c i e s and with the a x i s scores f o r the f i r s t 3 axes of the above o r d i n a t i o n of 20 quadrats are shown in Table 7. A few s i g n i f i c a n t c o r r e l a t i o n s were found suggesting that gross v e g e t a t i o n d i f f e r e n c e s w i t h i n a given f i e l d may to some extent be accounted f o r by u n d e r l y i n g edaphic v a r i a t i o n . DISCUSSION The unique management h i s t o r y of t h i s s e r i e s of . three d i f f e r e n t aged pastures provides an approximation of d i f f e r e n t c o e x i s t e n t stages of community e v o l u t i o n belonging to the same n a t u r a l sequence of development. T h i s i s c o r r o b o r a t e d by the t i m e - s e r i e s survey r e s u l t s f o r percentage cover. Time of p l a n t i n g ( i . e . age) i s assummed t h e r e f o r e to be a major v a r i a b l e d i s t i n g u i s h i n g the v e g e t a t i o n s t r u c t u r e of the three p a s t u r e s . I t must be recognized however that environmental c o n d i t i o n s (e.g. c l i m a t e , g r a z i n g . i n t e n s i t y ) at p a r t i c u l a r 56 TABLE 7. Product moment c o r r e l a t i o n c o e f f i c i e n t s f o r the 9 measured s o i l v a r i a b l e s with quadrat percentage cover f o r 10 of the most abundant s p e c i e s o v e r a l l at the study s i t e and with a x i s scores f o r the f i r s t 3 axes of the o r d i n a t i o n of 20 s e l e c t e d quadrats ( F i g . 6). a) 1977 pasture; b) 1958 pasture; c) 1939 pa s t u r e . (* s i g n i f i c a n t at the 5% l e v e l ; ** s i g n i f i c a n t at the 1% l e v e l ) . a) 1977 PASTURE pH Agropyron repens • 345 Dacty1i s glome rata .059 Festuca rubra .1*55* Holcus lanatus -.2*40 Lolium perenne .212 Phleum pratense .244 Poa compressa - . 3 6 2 T r i fo l ium repens - . 2 9 9 Ranunculus ac r i s - . 0 7 6 Taraxacum o f f i c i n a l e .371 Axis 1 - . 5 0 5 " Axis 2 -.048 Axis 3 .216 P N C CEC -.179 -.460* -.451* -.261 .502* .212 .266 .170 .185 -.642** -.524* -.345 -.258 -.109 -.107 -.149 .138 -.009 .075 .305 .342 -.105 -.147 -.267 - . 4 7 4 * .295 .164 -.069 -.221 .325 .379 .451 -.333 .025 .027 .001 .268 -.411 -.376 -.159 .022 .512* .503* .320 .042 -.374 -.421 -.477 .382 .127 .074 -.125 C a + + M g + + K + Na + -.251 -.048 -.072 .128 .364 .276 .361 .276 -.207 -.061 -.087 .088 -.406 -.505* -.085 -.368 .193 .070 .064 -.036 .092 .244 -.016 -.059 .030 .225 .173 .137 .216 .111 - . 1 8 7 .130 .026 .084 .490* .130 -.062 -.202 -.224 - .071 .185 -.026 .073 - . 1 0 8 - . 5 0 6 * -.419 .029 - . 2 7 0 • 359 . 5 0 7 - .075 .109 b) 1958 PASTURE pH Agropyron repens .063 Dactyl is glomerata .261 Festuca rubra .372 Holcus lanatus - .468* Lo l i urn perenne -.149 Phleum pratense -.048 Poa compressa .161 T r i f o l i urn repens .356 Ranunculus ac r i s -.609*- Taraxacum o f f i c i n a l e .306 Axis 1 .280 Axis 2 - .540* Axi s 3 -.234 P N C CEC .120 -.196 -.020 .141 .261 .086 -.142 .310 - . 0 4 3 -.388 -.092 -.201 - .615** .172 .059 - .530* .395 .138 .029 -.167 .117 .577** .585** .513* .489* .251 .201 .506* .478* -.160 -.221 .349 .298 .210 .361 -.342 .030 -.163 -.285 .292 .526* .115 -.012 . 5 5 7 * - . 4 1 9 .458 .166 -.218 .174 -.264 - . 266 -.440 C a + + M g + + K + N a + + .044 .483* -.099 -.001 .298 .341 .342 .164 -.365 -.031 -.096 .214 - .565* * -.295 -.206 -.406 .336 -.334 • 385 -.097 .661** -.068 .547* .361 .367 .347 .003 -.047 .646** .387 .199 .114 - .529* -.251 -.404 -.244 .232 .010 .201 -.180 .702** .340 .283 .130 -.201 -.185 -.002 -.414 -.004 -.172 -.242 -.035 c) 1939 PASTURE PH Agropyron repens .346 Dactyl Is glomerata .500* Festuca rubra .051 Holcus lanatus - .562** Loliurn perenne • 307 Phleum pratense -.194 Poa compressa .329 T r i f o l i urn repens .453* Ranunculus ac r i s -.336 Taraxacum o f f i c i n a l e .214 Axis 1 .569** Axis 2 .404 Axis 3 .096 P N C CEC .016 -.134 -.1 10 .037 -.132 -.390 -.303 -.035 -.222 .105 .083 .438 -.013 .296 .230 .249 -.334 -.141 -.016 .224 .407 .156 .159 • 113 .036 - . 0 2 3 .089 -.242 .368 .032 .017 .170 .080 .221 .197 -.092 -.349 -.172 -.146 .043 -.147 -.167 -.029 .042 .312 - . 2 1 0 -.195 -.231 -.197 .045 .189 -.243 Ca Mg + + K + Na + .117 .100 - . 1 7 8 .376 - . 1 2 8 .097 - . 3 0 8 .050 .135 - . 1 1 2 - . 0 3 0 .100 - . 0 7 9 .083 .146 .199 .078 .034 .165 - . 0 5 6 - . 1 1 0 .038 - . 1 3 3 - . 0 1 5 .203 - . 0 5 0 .012 - . 1 7 3 .193 - .111 -.045 .031 .159 .031 .149 .260 . 142 .245 - . 0 6 0 - . 1 2 8 .278 .034 -.056 -.024 -.042 - . 0 7 0 -.228 - . 1 8 3 .270 - . 0 1 7 .092 .054 60 times (e.g. duri n g and immediately f o l l o w i n g p l a n t i n g ) , as w e l l as the r e l a t i v e p r o p o r t i o n s of the s p e c i e s i n the p l a n t i n g mixture (Table 6) are l i k e l y to have been d i f f e r e n t i n the three f i e l d s . The cover of v a r i o u s s p e c i e s changes w i t h i n each f i e l d over the study p e r i o d and t h i s , not s u r p r i s i n g l y , i s at l e a s t p a r t i a l l y r e f l e c t i v e of seasonal growth p a t t e r n s . More impo r t a n t l y , the r e p r e s e n t a t i o n of v a r i o u s s p e c i e s d i f f e r s s u b s t a n t i a l l y among d i f f e r e n t aged f i e l d s ( F i g . 3). Some sp e c i e s show trends of i n c r e a s i n g r e l a t i v e abundance, and others d e c r e a s i n g r e l a t i v e abundance with i n c r e a s i n g f i e l d age. Poa compressa and T r i f o l i u m repens, the second and f o u r t h most abundant s p e c i e s i n the 1977 pasture, are the two most dominant s p e c i e s i n both o l d e r p a s t u r e s . There are no s p e c i e s (with the exception of T r i f o l i u m pratense and perhaps Lolium m u l t i f l o r u m ) that show a d e c l i n i n g t r e n d to t o t a l e x t i n c t i o n with i n c r e a s i n g f i e l d age. The i m p l i c a t i o n i s that a f t e r 43 years s i n c e the establishment of the 1939 pasture there have been no major s p e c i e s (now present i n the 1977 pasture) that have been t o t a l l y e l i m i n a t e d . Trends a l s o occur i n s e v e r a l s o i l v a r i a b l e s with i n c r e a s i n g f i e l d age ( F i g . 5) which may be e i t h e r r e s p o n s i b l e ' f o r , or a consequence of (or n e i t h e r ) , the trends i n s p e c i e s cover. Within-community c o r r e l a t i o n s of s p e c i e s cover with the s o i l v a r i a b l e s however was not g r e a t e r with i n c r e a s i n g pasture age. The o l d e s t (1939) pasture i n f a c t had n o t a b l y the fewest s i g n i f i c a n t c o r r e l a t i o n s of the three f i e l d s (Table 7). W i t h i n - community v e g e t a t i o n a l p a t t e r n s are t h e r e f o r e l e a s t accounted f o r by u n d e r l y i n g edaphic v a r i a t i o n i n the o l d e s t pasture 61 compared with the two younger p a s t u r e s . The o r d i n a t i o n r e s u l t s i n d i c a t e that the three study f i e l d s represent a sequence of pasture community development with respect to o v e r a l l v e g e t a t i o n a l p a t t e r n s . In p a r t i c u l a r , the youngest and o l d e s t pastures are the most d i f f e r e n t i n t h i s r espect and there i s more s t a b i l i t y over time i n the v e g e t a t i o n a l p a t t e r n s with i n c r e a s i n g pasture age ( F i g . 4). ' S t a b i l i t y ' here i s used i n the sense of community 'constancy' (Orians 1975). The youngest pasture showed the most v a r i a b i l i t y in community composition over the 33-month study p e r i o d and t h i s was represented by a d i r e c t i o n a l t rend i n the o r d i n a t i o n toward those quadrats i n o l d e r pastures ( F i g . 4). S u p e r f i c i a l l y the three study f i e l d s show no s t r i k i n g d i f f e r e n c e s ; t h e i r s p e c i es compositions are v i r t u a l l y i d e n t i c a l (although r e l a t i v e cover v a r i e s ) and d i f f e r e n c e s i n s o i l v a r i a b l e s are not l a r g e . A Simpson's d i v e r s i t y index (Peet 1974) was c a l c u l a t e d f o r each survey based on t o t a l percentage cover of each s p e c i e s per f i e l d (Table 8) and no s i g n i f i c a n t d i f f e r e n c e s were found amongst the three pastures over the study p e r i o d . In s p i t e of this., the data presented here on v e g e t a t i o n a l p a t t e r n s over time (Fig.4) exposes the three pastures as d i s t i n c t 'community e n t i t i e s ' , each d i s t i n g u i s h e d by i t s p e c u l i a r placement in a common developmental s e r i e s c h a r a c t e r i z e d by i n c r e a s i n g community constancy. I n c r e a s i n g community constancy however i s not accompanied by any s i g n i f i c a n t change i n s p e c i e s d i v e r s i t y . T h i s together with the lack of any c l e a r trends with i n c r e a s i n g pasture age f o r s o i l v a r i a b l e s on the quadrat o r d i n a t i o n s ( F i g . 6, Table 7) p o i n t s 62 TABLE 8. Simpson's d i v e r s i t y indexes (D) c a l c u l a t e d f o r each survey based on t o t a l percentage cover of each s p e c i e s per S 2 f i e l d . (D=1- E (p.) , where p. i s the p r o p o r t i o n of i n d i v i d u a l s i=i 1 1 of s p e c i e s i i n the community and S i s the number of s p e c i e s ) . The mean valu e s over the study p e r i o d were not s i g n i f i c a n t l y d i f f e r e n t amongst the three f i e l d s (P=0.3697) based on an a n a l y s i s of v a r i a n c e using a MIDAS program (Fox & Guire 1976). An F - t e s t f o r the homogeneity of v a r i a n c e s was not r e j e c t e d and a n a l y s i s was performed on untransformed data. SURVEY SURVEY PASTURE NO. DATE 1977 1958 1939 1 June 1 979 .859 .863 .822 2 Sept. 1 979 .809 .862 .832 3 Mar. 1980 .805 .790 .755 4 June 1980 .829 .845 .845 5 Sept. 1 980 .859 .842 .864 6 . Ma r . 1981 . .854 .831. .824 7. June 1981 .854 .868 .842 8 Sept. 1 981 .870 .888 .852 9 Mar. 1 982 .789 .844 .818 Mean .836 .848 .828 63 to the q u e s t i o n of the importance of b i o t i c f a c t o r s (e.g. g r a z i n g and neighbour i n t e r a c t i o n s ) i n determining the p r o p e r t i e s and development of neighbour r e l a t i o n s h i p s i n these p a s t u r e s . I t i s important to bear in mind that the three pastures have been exposed to d i f f e r e n t lengths of time of g r a z i n g ( i . e . s i n c e l a s t p l a n t e d ) . Grazing may provide an important s e l e c t i o n p r e s s u r e , e.g. i n a f f e c t i n g p l a n t growth h a b i t (Kemp 1937) or s p e c i e s r e l a t i v e abundance ( i f g r a z i n g i s p r e f e r e n t i a l ) . I t i s reasonable to expect that community s t r u c t u r e and dynamics may be a f f e c t e d by how long such s e l e c t i o n has been o p e r a t i n g . I t i s a l s o reasonable that the f i n d i n g s presented in t h i s chapter may be a t t r i b u t a b l e to the f a c t that the a n c e s t r a l components of o l d e r communities have had a longer h i s t o r y of b i o t i c i n t e r a c t i o n and hence a longer time for subsequent s e l e c t i v e f o r c e s to accumulate and generate b i o l o g i c a l accommodation. T h i s focuses a t t e n t i o n on the dynamics of i n t e r a c t i o n s o c c u r r i n g i n l o c a l neighbourhoods w i t h i n the community. Such m i c r o - s c a l e within-community, events are not adequately addressed at the t r a d i t i o n a l s c a l e of v e g e t a t i o n a l s u c c e s s i o n . The c r i t i c a l events occur at the l e v e l of i n d i v i d u a l response to s e l e c t i v e f o r c e s and i n v o l v e changes in g e n e t i c c o n s t i t u t i o n . These i s s u e s concerning neighbour i n t e r a c t i o n s are pursued in chapters to f o l l o w . Subsequent d i s c u s s i o n assumes however that s e l e c t i o n pressure r e l a t e d to both g r a z i n g and neighbour i n t e r a c t i o n s (e.g. competition) operate si m u l t a n e o u s l y and that t h e i r e f f e c t s may not be r e a d i l y separable. 64 CHAPTER 3 A QUALITATIVE MODEL FOR PASTURE COMMUNITY PRELIMINARY EVIDENCE FOR SPECIES INTERACTIONS USING CONTACT SAMPLING EVOLUTION: AND COEXISTENCE 65 INTRODUCTION Species i n t e r a c t i n nature. To the animal e c o l o g i s t t h i s i s o f t e n r e a d i l y observable, unprofound, and perhaps a t r u i s m . Yet between p l a n t s , s p e c i e s i n t e r a c t i o n i n nature i s o f t e n not immediately obvious. Most i n t e r e s t has centered on competition and e c o l o g i s t s even face an enigma i n the v i r t u a l i m p o s s i b i l i t y of e m p i r i c a l l y demonstrating that resources are r e a l l y l i m i t i n g in some communities. Only s t u d i e s on s p e c i e s removals from an area and the subsequent response of those l e f t u n d i sturbed have pro v i d e d some q u a n t i t a t i v e data (Sagar & Harper 1961, Putwain & Harper 1970, Pinder 1975, A l l e n & Forman 1976, A b u l - F a t i h & Bazzaz 1979, Fowler 1981). Some have c a l l e d f o r more a t t e n t i o n to be given to the r o l e of p r e d a t i o n and other d i s t u r b a n c e s (e.g. c l i m a t i c ) which are a l s o important f a c t o r s c o n t r o l l i n g p o p u l a t i o n s ( C o n n e l l 1975, Wiens 1977, Caswell 1978, Menge 1979). Again s t t h i s however, most s p e c i e s have an innate c a p a c i t y to produce many more o f f s p r i n g than i s necessary f o r p o p u l a t i o n replacement and yet remain on average f a i r l y constant i n p o p u l a t i o n s i z e . .: T h i s echoed paradigm of Malthus and -Darwin- supports the t a c i t assumption that resource, l i m i t a t i o n w i l l be common in nature and c o m p e t i t i v e i n t e r a c t i o n v i r t u a l l y i n e v i t a b l e . Dispute a r i s e s only with attempts to g e n e r a l i z e about the importance (or unimportance) of competition i n nature (e.g. Andrewartha & B i r c h 1954; see a l s o H a i r s t o n et ajL. 1960 versus Murdock 1966). I t i s more r e a l i s t i c to view competition as an i n t e r a c t i o n of v a r y i n g i n t e n s i t y determined by p r e v a i l i n g environmental c o n d i t i o n s , r a t h e r than an a l l - o r - n o t h i n g phenomenon (Pianka 1976). 66 A p e r s i s t e n t i n t e r a c t i o n between two s p e c i e s means that they are c o e x i s t i n g . S p a t i a l and temporal components are b u i l t i n t o the n o t i o n of c o e x i s t e n c e , both of which seem somewhat a r b i t r a r y ; c o e x i s t i n g s p e c i e s must i n some way be i n the same pl a c e together f o r some l e n g t h of time . The s p a t i a l component fo r mobile organisms i s very d i f f e r e n t from that f o r s e s s i l e organisms. Mobile organisms i n t e r a c t while a l t e r i n g t h e i r s p a t i a l r e l a t i o n s with respect to one another. The s p a t i a l component f o r s e s s i l e organisms however i s more e q u i v o c a l - the inherent q u e s t i o n e x i s t s as to what degree of p r o x i m i t y e l i c i t s an i n t e r a c t i o n between two permanently d i s p l a c e d i n d i v i d u a l s . In some v e g e t a t i o n types such as p a s t u r e s , s p e c i e s i n t e r a c t i o n s occur i n very l o c a l neighbourhoods i n v o l v i n g p l a n t s o f t e n i n p h y s i c a l c o n t a c t and the i n d i v i d u a l r a r e l y has any means of a d j u s t i n g i t s p r o x i m i t y to neighbours. Mack & Harper (1977) found that up to 69% of the v a r i a t i o n i n p l a n t weight and r e p r o d u c t i v e output of i n d i v i d u a l sand dune annuals can be accounted f o r by the neighbours present w i t h i n a r a d i u s of 2 cm. Sampling which records s p e c i e s presence/absence i n quadrats may not r e f l e c t the r e l e v a n t scale, of. i n t e r a c t i o n s , and a more a p p r o p r i a t e p e r s p e c t i v e may be to take a "plant's-eye-view" of the v e g e t a t i o n (Harper 1977a, 1977b, Turkington & Harper 1979a) based on p h y s i c a l c o n t a c t s between i n d i v i d u a l s . A p l o t l e s s method of sampling v e g e t a t i o n has been developed which enables i n v e s t i g a t i o n s of community s t r u c t u r e and v a r i a t i o n from the "plants-eye-view" i n terms of neighbour c o n t a c t s . I t was o r i g i n a l l y designed by Yarranton (1966) and f u r t h e r m o d i f i e d by Turkington (1975) and used r e c e n t l y i n 67 g r a s s l a n d s as the 'contact sampling method' (Turkington e_t a_l . 1977, Turkington & Harper 1979a, 1979b, Aarssen et a l . 1979). T h i s sampling scheme d e f i n e s a p r e c i s e s p a t i a l component for c o e x i s t e n c e i n p l a n t s . I t p r o v i d e s an unequivocal frame of r e f e r e n c e because neighbours are regarded as those l a t e r a l l y f i x e d i n d i v i d u a l s which p h y s i c a l l y contact above ground. Moreover, the method embodies a neighbourhood concept of community o r g a n i z a t i o n , i . e . i t i s s e n s i t i v e to the way that a p l a n t experiences i t s immediate neighbours and hence, non- randomness of i n t e r s p e c i f i c a s s o c i a t i o n i s d e t e c t e d at the s m a l l e s t p o s s i b l e s c a l e - the l o c a l neighbourhood. P l o t l e s s methods of sampling a v o i d the problems of s c a l e inherent i n determining a p p r o p r i a t e quadrat s i z e and shape for a p a r t i c u l a r v e g e t a t i o n type. The " s p e c i e s - j u x t a p o s i t i o n " method of r e c o r d i n g s p e c i e s sequence along a t r a n s e c t at small i n t e r v a l s (Stowe & Wade 1979) and the "neighbouring p o i n t a n a l y s i s " of Fowler & Antonovics (1981) are s i m i l a r p l o t l e s s methods a l s o o f f e r i n g these advantages. The l a t t e r study, c a r r i e d , out i n two d i f f e r e n t seasons, r e v e a l e d two d i s t i n c t groups of a s s o c i a t e d g r a s s l a n d s p e c i e s which d i f f e r e d i n t h e i r s e a s o n a l i t y of growth. Using the c o n t a c t sampling method in grass-legume communities, Turkington e_t a_l. (1977) found predominantly negative a s s o c i a t i o n amongst gr a s s e s , negative a s s o c i a t i o n amongst legumes and p o s i t i v e a s s o c i a t i o n between grasses and legumes. Turkington and Harper (1979b) found that in a 50-year-old pasture, a l l of the a s s o c i a t i o n s between T r i f o l i u m repens and each of e i g h t major grasses remained e i t h e r permanently p o s i t i v e or permanently negative over a 16-month 68 p e r i o d . Contact sampling goes f u r t h e r than any p r e v i o u s l y d e v i s e d technique f o r p r o v i d i n g q u a n t i t a t i v e data r e f l e c t i n g a measure of s p e c i e s c o e x i s t e n c e i n communities. Aarssen et a l . (1979), using the co n t a c t sampling method, i d e n t i f i e d three c a t e g o r i e s of s i g n i f i c a n t (P<0.05) i n t e r s p e c i f i c a s s o c i a t i o n i n pastures s t a b l e , seasonal and temporary. S a u l e i (1981) repo r t e d s i m i l a r types of a s s o c i a t i o n s i n abandoned f i e l d s . A s t a b l e a s s o c i a t i o n i s one which i s constant through time. A s t a b l e p o s i t i v e a s s o c i a t i o n d e t e c t e d by con t a c t sampling i s i n t e r p r e t e d as evidence that two s p e c i e s e x i s t f r e q u e n t l y w i t h i n each other's sphere of i n f l u e n c e (more o f t e n than expected by chance). A s t a b l e negative a s s o c i a t i o n on the other hand can be i n t e r p r e t e d as evidence f o r some measure of avoidance ( i n a non-cognizant sense) of i n t e r a c t i o n . T h i s however i s not evidence a g a i n s t c o e x i s t e n c e ; there may always be some i n d i v i d u a l s that enter i n t o one another's sphere of i n f l u e n c e . The present i n v e s t i g a t i o n uses the contact sampling method to provide a q u a n t i t a t i v e b a s i s f o r c o n s t r u c t i n g a p r e l i m i n a r y p i c t u r e of the nature and dynamics of f i n e - s c a l e s p e c i e s i n t e r a c t i o n s i n the study s i t e of d i f f e r e n t aged p a s t u r e s . Antonovics (1978) drew a t t e n t i o n to the lack of attempts to q u a n t i f y the process of community change i n p a s t u r e s . Aarssen et a l . (1979) and S a u l e i (1981) s t u d i e d i n t e r s p e c i f i c a s s o c i a t i o n s on three o c c a s s i o n s i n pastures which d i f f e r e d i n age ( s i n c e l a s t ploughed) by a range of 18 and 8 years r e s p e c t i v e l y . By comparison, the present i n v e s t i g a t i o n concerns three p a stures of which no two are l e s s than 20 years d i f f e r e n t 69 in t h e i r ages, and i n v o l v e s a longer term p e r i o d of study encompassing surveys i n 3 seasons over each of 3 y e a r s . The q u e s t i o n of c e n t r a l importance i s : What are the p a t t e r n s of change i n s p e c i e s a s s o c i a t i o n s d u r i n g pasture community development? The f o l l o w i n g h y p o thesis w i l l be t e s t e d : There are more ' s t a b l e ' (unchanging) a s s o c i a t i o n s i n o l d e r pastures than i n younger pa s t u r e s , whereas younger pastures have more a s s o c i a t i o n s which are r e l a t i v e l y s h o r t - l i v e d or 'temporary'. On the b a s i s of the r e s u l t s , a model i s proposed f o r the r o l e of changing p a t t e r n s of i n t e r s p e c i f i c a s s o c i a t i o n i n c h a r a c t e r i z i n g pasture community e v o l u t i o n . 'Community e v o l u t i o n ' i s used here not i n the sense of p a l e o e c o l g i c a l p r o g r e s s i o n (e.g. Gray et a_l . 1981) or community-level group s e l e c t i o n (e.g. Wilson 1976, 1980), but r a t h e r i n the sense that the p r o p e r t i e s of m u l t i - s p e c i e s assemblages may be viewed as community-level products of Darwinian n a t u r a l s e l e c t i o n (Whittaker & Woodwell 1971). Data on the p a t t e r n s of f i n e - s c a l e neighbour r e l a t i o n s h i p s w i l l a l s o provide a foundation f o r the experimental work re p o r t e d later., designed to . i d e n t i f y more p r e c i s e l y the- way(s) i n which c o m p e t i t i v e r e l a t i o n s change between s p e c i e s i n response to neighbour i n t e r a c t i o n s . METHODS Ve g e t a t i o n Sampling Data were c o l l e c t e d from 60 s y s t e m a t i c a l l y arranged 0.5 m x 0.5 m quadrats i n each of the three f i e l d s ( F i g . 1). Within each quadrat, contact-samples were taken at each of 25 70 s y s t e m a t i c a l l y arranged sampling p o i n t s , i . e . 1500 c o n t a c t - samples per f i e l d . There are s e v e r a l o p t i o n a l methods f o r data c o l l e c t i o n at each p o i n t . The method used here f o l l o w s the recommended s t r a t e g y of de Jong et a l . ( i n p r e s s ) . At each sampling p o i n t , the i n d i v i d u a l h i t by the v e r t i c a l downward p r o j e c t i o n of a sampling p i n i s the i n i t i a l sample sp e c i e s i ('point s p e c i e s ' ) . The sampling p o i n t i s subsequently removed from that l o c a t i o n to the nearest p o i n t of c o n t a c t with a neighbouring i n d i v i d u a l j ('contact mate'). If the c o n t a c t mate i s of i d e n t i c a l s p e c i e s to the p o i n t s p e c i e s ( i . e . j=i) then j i s r e j e c t e d and resampled u n t i l a contact mate of a d i f f e r e n t s p e c i e s from the p o i n t s p e c i e s i s determined, or u n t i l i t i s e s t a b l i s h e d that no d i f f e r e n t s p e c i e s i s i n contact with the p o i n t s p e c i e s . In the former case, the i n t e r s p e c i f i c contact p o i n t c l o s e s t to the o r i g i n a l sampling p o i n t i s u l t i m a t e l y chosen. Where the p o i n t s p e c i e s has no i n t e r s p e c i f i c c o n t a c t s or no c o n t a c t s whatever, or where there i s no p o i n t s p e c i e s , the e n t i r e sample i s r e j e c t e d and resampling occurs f o r another p o i n t s p e c i e s . No i n f o r m a t i o n i s gathered on i n t r a s p e c 1 f i c a s s o c i a t i o n i n t h i s sampling s t r a t e g y ; the procedure s e l e c t i v e l y focuses on neighbourhoods f e a t u r i n g i n t e r s p e c i f i c i n t e r a c t i o n . The advantage of t h i s i s that i t avoids the o f t e n formidable task i n g r a s s l a n d v e g e t a t i o n of t r y i n g to d i s t i n g u i s h whether, at the p o i n t of c o n t a c t , there are two d i f f e r e n t p l a n t s i n v o l v e d , or two d i f f e r e n t p a r t s of the same p l a n t . Each of the three pasture communities was surveyed using the above method on the same dates as the surveys f o r the 71 percentage cover of s p e c i e s , i . e three s p r i n g (March), three summer (June) and three f a l l (September) surveys were c a r r i e d out s t a r t i n g i n June 1979 and t e r m i n a t i n g with the l a s t survey in March 1982. Winter surveys were not p o s s i b l e due to i n t e r m i t t e n t s n o w f a l l . Data A n a l y s i s The hypothesis t e s t e d was that of random a s s o c i a t i o n amongst s p e c i e s . Randomness in t h i s context means that the number of c o n t a c t s between s p e c i e s does not d e v i a t e from random e x p e c t a t i o n . The p o i n t s p e c i e s and i t s c o n t a c t mate d e f i n e an ordered c o n t a c t p a i r ( i , j ) where i i s the s p e c i e s of the f i r s t s e l e c t e d p l a n t , and j that of the second. Table 9 i l l u s t r a t e s the a p p r o p r i a t e layout of the data i n a complete count matrix d i s p l a y i n g the number of each p a i r ) observed a f t e r N samples. Given the count matrix, previous p r a c t i c e has been to f o l d the matrix along the d i a g o n a l r e s u l t i n g in a t r i a n g u l a r a r r a y where c e l l ( i , j ) , i<j i s the aggregate number of c o n t a c t s between s p e c i e s i and j without d i s t i n g u i s h i n g between the order in which the p a i r s were determined. E a r l i e r s t u d i e s (Yarranton 1966, Turkington et a l . 1977, Turkington & Harper 1979a, 1979b) have analyzed such data using the usual but i n c o r r e c t procedure which i n c o r p o r a t e s a c h i - s q u a r e t e s t of a 2 X 2 contingency t a b l e . De Jong et a l . (1980) drew a t t e n t i o n to t h i s improper treatment of contact sampling data and presented a c o r r e c t method of a n a l y s i s f o r such f o l d e d contingency t a b l e s . Subsequently, de Jong et a l . ( i n press) designed a more s t r a i g h t f o r w a r d a n a l y s i s i n which the d i s t i n c t i o n between the 72 TABLE 9. Count matrix generated by contact sampling. f and s are r e s p e c t i v e l y the row and column t o t a l s f o r sp e c i e s k; n „ i s the number of ( i , j ) c o n t a c t p a i r s ; K i s the t o t a l number of s p e c i e s ; N i s the t o t a l number of contact-samples taken. F i r s t s e l e c t e d spec i e s (point s p e c i e s ) Second 1 s e l e c t e d s p e c i e s 2 3 . . . (contact K mate) T o t a l 1 - n i 2 n!3 ' ' • n IK f •l 2 "2 1 - n23 * * • n„ 2K f 2 3 • n31 n32 • • "n„ 3K • f 3 • R nK2 • « - fK T o t a l S 1 S 2 S • • 3 S K N TABLE -10. - Contingency t a b l e . f o r ( i , j ) i n t e r a c t i o n ; n o t a t i o n f o l l o w s that i n Table 9. F i r s t c h o i c e Second c h o i c e i j other T o t a l i - n . . 13 * i j n . . 3 i - £ r n u f3 other s.-n. . s.-n. . 3 13 N - f • - f • - s • - s • +n..+n.. 13 3 i N - f . - f . i 3 T o t a l s. 1 S . 3 N-S.-S. i 3 N 73 ordered p a i r s ( i , j ) and ( j , i ) i s maintained and the data are t a b u l a t e d i n an unfolded matrix as i n Table 9. V a r i a t i o n s i n growth forms and percentage cover amongst the d i f f e r e n t s p e c i e s d i c t a t e that f i r s t and second c h o i c e p r o b a b i l i t i e s w i l l be d i f f e r e n t under the above s p e c i f i e d sampling s t r a t e g y . As de Jong et a l . ( i n press) have argued, t h i s c a l l s f o r the p a r a m e t e r i z a t i o n of the random a s s o c i a t i o n hypothesis i n terms of two s e t s of parameters: the f i r s t r e l a t e d to the r e l a t i v e cover of the v a r i o u s s p e c i e s , and a second set r e l a t e d to growth d e n s i t i e s at contact l e v e l . T h i s second set of parameters accounts f o r v e r t i c a l s t r u c t u r e i n the v e g e t a t i o n which may be assumed v i r t u a l l y n e g l i g i b l e i n some v e g e t a t i o n types such as l i c h e n - b r y o p h y t e communities (e.g. Yarranton 1966) or grazed g r a s s l a n d s as i n the present study. In such s i t u a t i o n s c o n t a c t s e s s e n t i a l l y occur i n a l a t e r a l plane which i m p l i e s that the non- zero second choice p r o b a b i l i t i e s are p r o p o r t i o n a l to the f i r s t c h o i c e p r o b a b i l i t i e s . T h i s has been c a l l e d the " p r o p o r t i o n a l i t y h y p o t h e s i s " a f t e r de Jong e_t a l . ( i n p r e s s ) . D e t a i l s of data a n a l y s i s for. random a s s o c i a t i o n are presented there and w i l l only be summarized below.. The p r o b a b i l i t y of s e l e c t i n g the ordered p a i r ( i , j ) , not assuming p r o p o r t i o n a l i t y , i s given by P i q j / ( l - q i ) , i , j = l , 2 , . . . , K ; i ^ j where p i s the r e l a t i v e cover frequency of s p e c i e s k i n the community and q i s the r e l a t i v e growth d e n s i t y of s p e c i e s k at the v e r t i c a l p o s i t i o n i n the canopy where con t a c t occurs. T h i s t e s t i s c o n d i t i o n a l on a l l other a s s o c i a t i o n s assumed random. T h i s i s o f t e n a r e s t r i c t i v e assumption which can be avoided 74 under the p r o p o r t i o n a l i t y h y p o t h e s i s . T h i s h y p othesis s t a t e s that q = p and the p r o b a b i l i t y of s e l e c t i n g the ordered p a i r ( i , j ) becomes p . p . / ( l - p . ) • i j i The e s t i m a t i o n of the maximum l i k e l i h o o d expected c e l l counts, e under e i t h e r random p a i r i n g model i s accomplished using an i t e r a t i v e procedure o u t l i n e d by De Jong et a l . ( i n p r e s s ) . To measure the d e v i a t i o n from random a s s o c i a t i o n the Pearson goodness of f i t s t a t i s t i c was used: X 2 = E E ( n ; . - e . . ) 2 / e . . i j ! 3 ID ID S i g n i f i c a n t l y l a r g e values of c h i - s q u a r e argues a g a i n s t the h ypothesis used to c o n s t r u c t the e i j v a l u e s , and hence i s evidence f o r non-randomness in the data matrix as a whole. Because the c hi-square technique i s a 'large-sample' method, i t cannot be l e g i t i m a t e l y a p p l i e d i n s i t u a t i o n s where there are few o b s e r v a t i o n s . Conventional p r a c t i c e has set a somewhat a r b i t r a r y l i m i t that the expected number of counts in each c e l l should be at l e a s t f i v e . A c c o r d i n g l y , rare s p e c i e s , o c c u r r i n g in l e s s than 40, contact-samples i n t o t a l , were excluded.from the a n a l y s i s . For i n v e s t i g a t i o n of p a r t i c u l a r p a i r w i s e a s s o c i a t i o n s , the whole matrix of counts was f i r s t c o l l a p s e d i n t o c e l l s d e f i n e d by Table 10. Expected c e l l counts corresponding to t h i s t a b l e were c a l c u l a t e d u sing an i t e r a t i v e procedure which mimics that used for the o v e r a l l t e s t and again, a c h i - s q u a r e s t a t i s t i c was used to i n d i c a t e departures from randomness. For c e r t a i n s p e c i e s p a i r s of p a r t i c u l a r i n t e r e s t , an index of a s s o c i a t i o n was c a l c u l a t e d as C=1-P where P i s the s i g n i f i c a n c e p r o b a b i l i t y from 75 the c h i - s q u a r e t e s t . The s i g n ( p o s i t i v e or negative) of s i g n i f i c a n t a s s o c i a t i o n s was obtained from the sig n of the 'standardized r e s i d u a l s ' , being the components of c h i - s q u a r e : (n. . -e . . ) /Je. . iD iD 1D • The index C t h e r e f o r e ranges between +1.0 (maximum p o s i t i v e a s s o c i a t i o n ) and -1.0 (maximum negative a s s o c i a t i o n ) . S i g n i f i c a n t a s s o c i a t i o n s (at l e s s than the 5% l e v e l ) occur when O + 0.95 and C<-0.95. RESULTS For a l l 27 data sets the o v e r a l l c h i - s q u a r e t e s t i n d i c a t e d g l o b a l departures from randomness. A c c o r d i n g l y , i t was decided that an exact t e s t for i n d i v i d u a l p a i r w i s e a s s o c i a t i o n s which assumes that other p a i r s are random was i n a p p r o p r i a t e . The p r o p o r t i o n a l i t y hypothesis does not invoke t h i s assumption (de Jong et a_l. , i n press) and was t h e r e f o r e used to d e t e c t s i g n i f i c a n t a s s o c i a t i o n s f o r a l l p a i r w i s e combinations. S i g n i f i c a n t (P<0.05) p a i r w i s e a s s o c i a t i o n s , together with t h e i r s i g n s ( p o s i t i v e ' or negative), are-, l i s t e d , i n Table 1 "1 . f or a l l surveys Table 11 shows that a s s o c i a t i o n s between grasses are predominantly negative and t h i s i s more pronounced in o l d e r p a s t u r e s . An exception i s Agropyron repens - Poa compressa which shows p e r i o d i c p o s i t i v e a s s o c i a t i o n . S i g n i f i c a n t p o s i t i v e a s s o c i a t i o n s t y p i c a l l y i n v o l v e d a grass and a non-grass, e s p e c i a l l y T r i f o l i u m repens and Taraxacum of f i c i n a l e . Most s i g n i f i c a n t a s s o c i a t i o n s were 'temporary', o c c u r r i n g i n only one or a few s u c c e s s i v e surveys, and then d i s a p p e a r i n g 76 TABLE 11. Summmary of s i g n i f i c a n t (P<0.05) i n d i v i d u a l p a i r w i s e a s s o c i a t i o n s in the three pastures i n each survey. Each value i s the a t t a i n e d s i g n i f i c a n c e l e v e l (to 3 decimal p l a c e s ) when t e s t i n g f o r i n d i v i d u a l i n t e r s p e c i f i c a s s o c i a t i o n together with the s i g n (+ or -) obtained from the s t a n d a r d i z e d r e s i d u a l s . Species names are a b b r e v i a t e d to t h e i r generic name only except in cases where more than one s p e c i e s i s represented f o r a genus. Complete s p e c i e s names are a v a i l a b l e i n Table 5. 1977 PASTURE SURVEY NO. SEASON AGROPYRON AGROPYRON AGROPYRON AGROPYRON AGROPYRON AGROPYRON AGROPYRON AGROPYRON AGROPYRON AGROPYRON AGROSTIS - AGROSTIS - AGROSTIS - AGROSTIS - AGROSTIS - AGROSTIS - DACTYLIS - DACTYLIS - DACTYLIS - DACTYLIS - DACTYLIS - DACTYLIS - - DACTYLIS - HOLCUS - LOLIUM - PHLEUM - PLANTAGO LANCEOLATA - POA COMPRESSA - RANUNCULUS - TARAXACUM - TRIFOLIUM PRATENSE - TRIFOLIUM REPENS DACTYLIS LOLIUM POA COMPRESSA RANUNCULUS TARAXACUM TRIFOLIUM REPENS FESTUCA HOLCUS LOLIUM PHLEUM PLANTAGO LANCEOLATA POA COMPRESSA 1 2 3 4 5 6 7 8 9 SUM FALL SPR SUM FALL SPR SUM FALL SPR K005 -,000 .002 000 -.„039 -.000 -.000 + .000 + .023 + .046 + .000 -.030 -.034 -.036 + „007 -.000 + .018 + .005 -.007 -.003 -.000 .004 -.001 .050 -.001 .002 + .038 + .000 -.006 .001 -.000 .021 -.001 +.000 +.014 + .043 +.001 +.000 +.005 +.000 -.034 -.042 -.019 + .000 + .000 -.000 -.001 -.000 -.004 -.000 -.000 +.000 +.000 -.004 + .000 + .000 -.000 -.024 1977 PASTURE SURVEY NO. (CONT.) SEASON DACTYLIS - RANUNCULUS . DACTYLIS - TARAXACUM DACTYLIS - TRIFOLIUM PRATENSE DACTYLIS - TRIFOLIUM REPENS FESTUCA - POA COMPRESSA FESTUCA - TARAXACUM FESTUCA - TRIFOLIUM REPENS HOLCUS - LOLIUM HOLCUS - POA COMPRESSA HOLCUS - RANUNCULUS HOLCUS - TARAXACUM HOLCUS - TRIFOLIUM REPENS LOLIUM - PHLEUM LOLIUM - PLANTAGO LANCEOLATA LOLIUM - POA COMPRESSA LOLIUM - RANUNCULUS LOLIUM - .TARAXACUM LOLIUM - TRIFOLIUM REPENS PHLEUM - PLANTAGO LANCEOLATA PHLEUM - POA COMPRESSA PHLEUM - TARAXACUM PHLEUM - TRIFOLIUM REPENS 1 2 3 4 5 6 7 8 9 SUM FALL SPR SUM FALL SPR SUM FALL SPR -,003 + .005 .002 +.000 -.000 .036 +.000 .017 + .010 .005 +.001 + .003 -.043 --009 +.000 -.005 +.000 + .012 + .000 + .038 .007 -.001 +.015 + .000 -.009 -.025 .000 + .003 + .002 -.001 -.043 +.000 +.000 .005 -.005 -.000 -.001 -.000 .000 +.038 -.000 .035 + .020 -.004 +.013 +.000 +.001 +.045 +.045 +.039 +.000 +.002 +.000 + .000 -.005 +.013 +.019 +.000 +.003 +.000 +.000 -.001 -.001 -.015 + .009 +.001 +.003 + 043 +.,009 1977 PASTURE SURVEY NO. : (CONT.) SEASON : 1 SUM 2 FALL 3 SPR 4 SUM 5 FALL 6 SPR 7 SUM 8 FALL 9 SPR POA COMPRESSA - PLANTAGO LANCEOLATA + .000 POA COMPRESSA - RANUNCULUS + .000 + .000 POA COMPRESSA - TARAXACUM + .028 + .000 + .000 -.027 + .001 + .001 POA COMPRESSA - TRIFOLIUM REPENS + .002 + .000 + .000 + .001 -.000 TRIFOLIUM REPENS - P. LANCEOLATA + .003 TRIFOLIUM REPENS - RANUNCULUS + .004 + .001 + .001 TRIFOLIUM REPENS - TARAXACUM + .019 + .006 + .009 TRIFOLIUM REPENS - T. PRATENSE -.002 PLANTAGO LANCEOLATA - RANUNCULUS + .048 PLANTAGO LANCEOLATA - TARAXACUM + .000 + .000 + .014 1958 PASTURE SURVEY NO. : 1 2 3 4 5 6 7 8 9 SEASON : SUM FALL SPR SUM FALL SPR SUM FALL SPR AGROPYRON - DACTYLIS -,028 -.000 _ .007 -.000 -.072 AGROPYRON - FESTUCA -.029 -.005 • AGROPYRON - HOLCUS -.046 -.013 -.007 -.000 -.000 AGROPYRON - PHLEUM -.026 - .022 -.006 AGROPYRON - POA COMPRESSA -.001 -.045 + .000 + .000 + .000 AGROPYRON - RANUNCULUS + .043 AGROPYRON - TARAXACUM + .002 + .000 + .000 AGROPYRON - TRIFOLIUM REPENS + .000 + .000 + .009 + .002 -.047 AGROSTIS - DACTYLIS -.019 -.016 AGROSTIS .- HOLCUS -.002 AGROSTIS - RANUNCULUS + .003 AGROSTIS - TARAXACUM + .014 AGROSTIS - TRIFOLIUM REPENS + .047 + .007 ANTHOXANTHUM - DACTYLIS -.039 -.022 -.031 ANTHOXANTHUM - FESTUCA + .037 + .000 ANTHOXANTHUM - PLANTAGO LANCEOLATA + .000 ANTHOXANTHUM - POA COMPRESSA -.034 ANTHOXANTHUM - RANUNCULUS + .001 ANTHOXANTHUM - TRIFOLIUM REPENS + .015 + .000 DACTYLIS - FESTUCA -.000 - . 000 -.000 -.002 -.000 -.000 -„000 -.000 -.000 DACTYLIS - HOLCUS -„000 -.000 - .000 -.049 -.037 -.000 -.000 -.000 -.000 DACTYLIS - LOLIUM -.034 -.000 -.001 -.000 + .001 1958 PASTURE SURVEY NO. (CONT.) SEASON DACTYLIS - PHLEUM DACTYLIS - PLANTAGO LANCEOLATA DACTYLIS - POA COMPRESSA DACTYLIS - POA TRIVIALIS DACTYLIS - RANUNCULUS DACTYLIS - TARAXACUM DACTYLIS - TRIFOLIUM REPENS FESTUCA - HOLCUS FESTUCA - LOLIUM FESTUCA - PLANTAGO LANCEOLATA FESTUCA - POA COMPRESSA FESTUCA - RANUNCULUS FESTUCA - TARAXACUM FESTUCA - TRIFOLIUM REPENS HOLCUS - LOLIUM HOLCUS - PHLEUM HOLCUS - POA COMPRESSA HOLCUS - RANUNCULUS HOLCUS - TARAXACUM HOLCUS - TRIFOLIUM REPENS LOLIUM - PHLEUM LOLIUM - PLANTAGO LANCEOLATA 1 2 3 4 5 6 7 8 9 SUM FALL SPR SUM FALL SPR SUM FALL SPR -.000 + .000 + .013 -.000 -.009 .000 .000 -.000 + .000 +.007 +.000 + .007 -.011 -.000 -.005 -.000 +.000 +.000 +.002 -.002 -.000 +.000 +.000 -.005 +.008 + .000 +.006 +.000 + .000 +.000 +.009 +.000 +.022 + .007 + .008 +,002 +.010 -039 -.000 -.009 -.001 -.009 +.001 -.000 +.000 +.000 + .000 +.012 +.015 + .002 -.021 -.023 -.004 +. 001 +.013 -.000 +.017 +.006 + .004 -.002 + .000 -.031 -.005 + .000 -.008 + .000 -.001 -.034 + .002 -.016 + .001 -.046 + .000 + .026 + .049 + .000 + .000 -.000 + .000 + .000 -.012 -.000 + .000 oo + „006 1958 PASTURE SURVEY NO. (CONT.) SEASON LOLIUM - POA COMPRESSA LOLIUM - RANUNCULUS LOLIUM - TARAXACUM LOLIUM - TRIFOLIUM REPENS PHLEUM - POA COMPRESSA PHLEUM - POA TRIVIALIS PHLEUM - RANUNCULUS PHLEUM - TARAXACUM PHLEUM - TRIFOLIUM REPENS POA COMPRESSA - POA TRIVIALIS POA COMPRESSA - P. LANCEOLATA POA COMPRESSA - TARAXACUM POA COMPRESSA - TRIFOLIUM REPENS TRIFOLIUM REPENS - P. LANCEOLATA TRIFOLIUM REPENS - RANUCULUS TRIFOLIUM REPENS - TARAXACUM PLANTAGO LANCEOLATA - RANUNCULUS 1 2 3 4 5 6 7 8 9 SUM FALL SPR SUM FALL SPR SUM FALL SPR + .007 +.001 +.031 +.000 -.000 + .000 + .001 -.030 +.014 +.000 -.022 +.013 +.025 +.000 +.000 +.000 +.000 + .000 -.035 -.005 + .018 +,000 +.004 +.000 -.015 +.008 + .006 +.000 +.001 +.000 +.000 +.000 + .047 -.020 -.024 +.000 + .001 +.005 +.000 +.023 + .012 +.012 +.017 +.004 -.028 +.000 +.006 +.007 +.000 +.002 +.002 . + .000 +.013 +.036 +.000 1939 PASTURE SURVEY NO. SEASON AGROPYRON - CIRSIUM AGROPYRON - FESTUCA AGROPYRON - HOLCUS AGROPYRON - LOLIUM AGROPYRON - PHLEUM AGROPYRON - POA COMPRESSA AGROPYRON - TARAXACUM AGROPYRON - TRIFOLIUM REPENS AGROSTIS - HOLCUS AGROSTIS - TRIFOLIUM REPENS DACTYLIS - CIRSIUM DACTYLIS - HOLCUS DACTYLIS - LOLIUM DACTYLIS - PHLEUM DACTYLIS - POA COMPRESSA DACTYLIS - TARAXACUM DACTYLIS - TRIFOLIUM REPENS FESTUCA - POA COMPRESSA FESTUCA - RANUNCULUS FESTUCA - TARAXACUM FESTUCA - TRIFOLIUM REPENS HOLCUS - LOLIUM 1 2 3 4 5 6 7 8 9 5 U M F A L L S P R SUM FALL SPR SUM FALL SPR +.030 +.040 -,001 -.023 -.031 -.007 + .009 -.013 -.000 -.011 + .043 + .000 -.000 + .000 + .037 -,,017 + .000 + .032 -.020 -.031 -.005 +.000 +.021 +.001 +.001 + .005 + .037 -.006 -.002 -.000 -.008 -.022 -.018 -.018 -.039 + .014 +.026 +.008 -.001 -.005 -.028 + .000 +.012 +.000 + .039 -.047 -.041 -.006 -.007 .018 -.006 +.000 +.001 +.007 +.000 + .003 -.008 -.008 -.005 + .021 + .001 .001 -.000 -.039 + .046 +.000 +.000 + .043 -.000 -.000 1939 PASTURE SURVEY NO. (CONT.) SEASON HOLCUS - PHLEUM HOLCUS - POA COMPRESSA HOLCUS - RANUNCULUS HOLCUS - TARAXACUM HOLCUS - TRIFOLIUM REPENS LOLIUM -CIRSIUM LOLIUM - POA COMPRESSA LOLIUM - TARAXACUM LOLIUM - TRIFOLIUM REPENS PHLEUM - POA COMPRESSA PHLEUM - RANUNCULUS PHLEUM - TARAXACUM PHLEUM - TRIFOLIUM REPENS POA COMPRESSA - RANUNCULUS POA COMPRESSA - TARAXACUM POA COMPRESSA - TRIFOLIUM REPENS TRIFOLIUM REPENS - CIRSIUM TRIFOLIUM REPENS - RANUNCULUS TRIFOLIUM REPENS - TARAXACUM RANUNCULUS - TARAXACUM 1 2 3 4 5 6 7 8 9 SUM FALL SPR SUM FALL SPR SUM FALL SPR -.037 -.023 +.001 +.000 +.000 -.000 + .000 -.000 + .027 + .000 -.012 + .000 + .000 +.000 +.000 + .000 + .000 -.046 + .026 -.000 -.000 + .000 + .012 -.020 + .000 + .000 -.041 -.023 +.013 -.020 -.000 +.001 -.011 -.000 + .026 + .010 + .022 -.005 + .032 -.006 + .035 + .000 -.017 -.000 +.003 +.026 +.001 +.000 -.000 -.000 -.000 -.001 +.016 +.026 +.015 +.000 +.000 +.000 +.016 +.000 + .022 +.003 -.014 +.000 +.000 -.022 -.008 -.037 + .048 + .024 +.000 +.000 +.028 +.021 +.014 CO 85 and sometimes reappearing in l a t e r surveys. The o l d e s t pasture had notably fewer temporary a s s o c i a t i o n s than the two younger pastures (Table 12). Some s p e c i e s p a i r s a l t e r n a t e d between p o s i t i v e and negative a s s o c i a t i o n and t h i s was e s p e c i a l l y evident i n the youngest p a s t u r e . The youngest pasture had no s i g n i f i c a n t p a i r e d a s s o c i a t i o n s which were c o n s i s t e n t l y p o s i t i v e or negative over the 33-month study p e r i o d . However, the 1958 pasture had 4 such ' s t a b l e ' a s s o c i a t i o n s and the 1939 pasture had 6 s t a b l e a s s o c i a t i o n s ( F i g . 7). The occurrence of i n c r e a s i n g s t a b i l i t y or constancy of p o s i t i v e and negative a s s o c i a t i o n between p a r t i c u l a r s p e c i e s p a i r s with i n c r e a s i n g pasture age i s f u r t h e r i l l u s t r a t e d i n F i g u r e 8. P a r t i c u l a r l y notable i s the development with i n c r e a s i n g pasture age, of s t a b l e negative a s s o c i a t i o n between 4 d i f f e r e n t p a i r s of grass s p e c i e s • ( F i g . 8 a,c,e,h) and the development of more s t a b l e p o s i t i v e a s s o c i a t i o n between T r i f o l i u m repens and 5 d i f f e r e n t grasses ( F i g . 8 b , d , g , i , j ) . Likewise, p a t t e r n s of seasonal a s s o c i a t i o n show a s i m i l a r t r e n d r e f l e c t i n g a tendency of i n c r e a s i n g community constancy. A seasonal a s s o c i a t i o n can been seen developing between Holcus lanatu s and Poa compressa which were s i g n i f i c a n t l y negative i n a s s o c i a t i o n i n a l l three s p r i n g surveys in the 1939 and 1958 p a s t u r e s , but showed no seasonal p a t t e r n of a s s o c i a t i o n i n the youngest pasture ( F i g . 8 f ) . In the 1977 p a s t u r e , glomerata and T^ repens were p o s i t i v e l y a s s o c i a t e d i n a l l three f a l l surveys but t h i s seasonal p a t t e r n was not present in the two o l d e r p a stures where a s s o c i a t i o n was p r o g r e s s i v e l y l e s s v a r i a b l e ( F i g . 8d). The same trend was found for L^ perenne and T^ 86 FIGURE 7. St a b l e p a i r w i s e a s s o c i a t i o n s ( p o s i t i v e - s o l i d l i n e s , and negative - broken l i n e s ) d e t e c t e d i n a) the 1958 pasture, and b) the 1939 pasture. Each a s s o c i a t i o n was s i g n i f i c a n t (P<0.05) in a l l nine surveys (except P_̂  compressa - T. o f f i c i n a l e i n a) with P=0.070 i n survey 5, and p_j_ glomerata H. l a n a t u s i n b) with P=0.083 i n survey 3 ) . 87 1958 PASTURE TRIFOLIUM POA TARAXACUM REPENS COMPRESSA OFFICINALE FESTUCA , / DACTYLIS // HOLCUS RUBRA GLOMERATA LANATUS 1939 PASTURE DACTYLIS GLOMERATA N TRIFOLIUM 88 FIGURE 8. Course l i n e s showing the degree of a s s o c i a t i o n f o r s e l e c t e d i n d i v i d u a l s p e c i e s p a i r s versus the number of years s i n c e the sampled f i e l d was sown. Survey data from the three study f i e l d s over a p e r i o d of 33 months from June 1979 to March 1982 are represented i n each graph. P l o t t e d v a l u e s are a s s o c i a t i o n indexes c a l c u l a t e d as C=1-P with the a p p r o p r i a t e s i g n of the s t a n d a r d i z e d r e s i d u a l a t t a c h e d and where P i s the s i g n i f i c a n c e p r o b a b i l i t y l e v e l f o r the p a r t i c u l a r p a i r w i s e a s s o c i a t i o n . Closed c i r c l e s represent s i g n i f i c a n t a s s o c i a t i o n s at P<0.05 (C>0.95 or C<-0.95). The months i n which surveys were conducted are i n d i c a t e d as M=March, J=June, S=September. 89 PASTURE 1958 PASTURE 1939 PASTURE a) Agropyron repens — Holcus lana t u s —i i i i i i i i—i —i—i—i— i—i— i—i—i—i J S M J S M J S M J S M J S M J S M -1—I 1 1—I 1 S M J S M I I 1—I 1 1—I—I 1 J S M J S M J S M Agropyron repens — T r i f o l i u m repens ~i—i—i—i—i—i—i—i—i J S M J S M J S M c) D a c t y l i s glomerata Holcus lana t u s L 1 1 J s ^—r-M J 21 22 23 40 41 42 YEARS SINCE PLANTING 90 PASTURE 1958 PASTURE 1939 PASTURE d) Dactyl i s glomerat. T r i f o l i u m repens I I I I I 1—I 1 1 1—I 1—I—I 1 1—I 1 M J S M J S M.I S M J S M J S M .1 S M 4 21 22 23 40 41 42 YEARS SINCE PLANTING 91 1977 PASTURE 1958 PASTURE 1939 PASTURE 1 .00 T 0.80 0.60 -1 « 0.40 c 0.20 z z 0 < ~ -0.20 X •r. -0.40 -0.60 - -0.50- - 1 . 00 e) Ho]cus l a n a t u s — T r i f o l i u m repens —1—1—1 1—1—i— M J S M J S -1—r— J s -1—1—1— M J S -1—1 1— M J S 92 1977 PASTURE 1958 PASTURE 1939 PASTURE 93 repens which were s i g n i f i c a n t l y negative in a s s o c i a t i o n i n the three s p r i n g surveys f o r the 1977 pasture, but a s s o c i a t i o n was l e s s v a r i a b l e i n the two o l d e r pastures and was p o s i t i v e and s t a b l e i n the 1939 p a s t u r e . P_;_ compressa and Taraxacum o f f i c i n a l e showed a s t a b l e behaviour of p o s i t i v e a s s o c i a t i o n i n the 1958 pasture but a temporary p a t t e r n of a s s o c i a t i o n i n the other two pastures ( F i g . 8k). The o v e r a l l trend i s one of i n c r e a s i n g s t a b i l i t y of a s s o c i a t i o n s between s p e c i e s i n p r o g r e s s i v e l y o l d e r f i e l d s . DISCUSSION The r e a c t i o n of i n d i v i d u a l s to the presence of other organisms i s c r u c i a l to the understanding of the ecology of both p o p u l a t i o n s and communities. The manner of coping with the b i o t i c environment c o n s i t i t u t e s a major p a r t of the e n v i r o n m e n t a l l y - i n d u c e d genetic changes in many sp e c i e s (Cantlon 1968). In r e f e r r i n g to s p e c i e s assemblages over broad s p a t i a l and temporal s c a l e s , Whittaker (1975) commented, "Species a s s o c i a t i o n s with other s p e c i e s are : predominantly loose and changeable, and community e v o l u t i o n i s n e t - l i k e i n the sense that s p e c i e s are v a r i o u s l y combined and recombined i n t o communities i n e v o l u t i o n a r y time". No s p e c i a l d i s t i n c t i o n of d u r a t i o n need be imposed however for ' e v o l u t i o n a r y time'; e v o l u t i o n a r y changes may occur on almost any time s c a l e (Antonovics 1976a). The r e s u l t s presented provide evidence f o r the t r a n s i e n t nature of i n t e r s p e c i f i c a s s o c i a t i o n d i s c u s s e d by Whittaker, on a very l o c a l s c a l e . D e t a i l e d s t u d i e s of developmental changes i n 94 l o c a l p a t t e r n s of a s s o c i a t i o n i n communities, from the " p l a n t ' s - eye-view", have been recent and few i n number (Aarssen et a l . 1979, Turkington & Cavers 1979, S a u l e i 1981). The present study i n v o l v e s more i n t e n s i v e sampling over a longer p e r i o d of i n v e s t i g a t i o n i n pastures which d i f f e r more widely i n age than in previous s t u d i e s . The r e s u l t s are c o n s i s t e n t with the f i n d i n g s i n e a r l i e r work, i . e . younger communities have a preponderance of i n t e r s p e c i f i c a s s o c i a t i o n s which are temporary i n nature d u r i n g the study p e r i o d and o l d e r communities have more a s s o c i a t i o n s which p e r s i s t e s s e n t i a l l y unchanged over the same time p e r i o d than do younger communities. Table 12 summarizes the temporal p a t t e r n s of a s s o c i a t i o n s in the three p a s t u r e s . TABLE 12. Numbers of s i g n i f i c a n t temporary, seasonal and s t a b l e a s s o c i a t i o n s (both p o s i t i v e and negative) d e t e c t e d in the three d i f f e r e n t aged p a s t u r e s . YEAR OF NUMBER OF SIGNIFICANT ASSOCIATIONS (P<0.05) PLANTING Temporary Seasonal S t a b l e 1977. 52 2 0 1958 ' 57 0 4 1939 35 1 6 Apparently, a s s o c i a t i o n s between s p e c i e s become l e s s 'loose and changeable' and pasture community e v o l u t i o n tends toward a s t a b i l i t y i n s p e c i e s i n t e r r e l a t i o n s h i p s . The developmental trends toward s t a b i l i t y i n i n t e r s p e c i f i c a s s o c i a t i o n s ( F i g . 11) 95 p a r a l l e l those d i r e c t i o n a l trends i n the o r d i n a t i o n of quadrat percentage cover data ( F i g . 4, Chapter 2). The data on neighbour c o n t a c t s however t r a n s l a t e s the gross v e g e t a t i o n a l p a t t e r n s from quadrat sampling i n t o an i d e n t i f i c a t i o n of the p a r t i c u l a r neighbours r e p r e s e n t i n g the i n d i v i d u a l p l a n t ' s experience of i t s immediate b i o t i c environment. The general p i c t u r e i s that of a t r a n s i t i o n from an e s s e n t i a l l y random assemblage of neighbourhoods i n which the p o s i t i o n a l r e l a t i o n s h i p of c o n s i t u e n t members i s i n a constant s t a t e of f l u x , to a more f i x e d community matrix i n which the most proximate i n t e r s p e c i f i c neighbours ( i . e . i n p h y s i c a l contact) share a more or l e s s permanent address. The r e s u l t s may be best i n t e r p r e t e d by p l a c i n g them i n t o a t h e o r e t i c a l and e v o l u t i o n a r y context f o r the pasture community. A popular view of community e v o l u t i o n was propounded by Whittaker and Woodwell (1971, p. 137): "The community i s the context of s p e c i e s e v o l u t i o n ... the e v o l u t i o n of a community must e n t a i l ' p a r a l l e l ' or coadaptive e v o l u t i o n of the community's s p e c i e s . The community i s an , assemblage of i n t e r a c t i n g and c o e v o l v i n g s p e c i e s . . . . Through t h i s e v o l u t i o n there w i l l appear ad a p t a t i o n to environment f o r the community as w e l l as the s p e c i e s . . . . Community l e v e l c h a r a c t e r i s t i c s a l s o w i l l e v o l v e " . Community e v o l u t i o n in t h i s p e r s p e c t i v e means that community-level c h a r a c t e r i s t i c s (e.g. d i v e r s i t y , dominance, physiognomy, t r o p h i c s t r u c t u r e , s t a b i l i t y , s p a t i a l p a t t e r n , coadaptive a s s o c i a t i o n s or networks, e t c . ) w i l l tend to f o l l o w c e r t a i n d e d u c i b l e and r e c o g n i z a b l e trends through time d i c t a t e d by the f a c t that they are community-level products of 96 e v o l u t i o n and i n t e r a c t i o n at the s p e c i e s l e v e l (Whittaker & Woodwell 1971). Cody & Diamond (1975) echoed a s i m i l a r sentiment arguing that "... the observed p a t t e r n s i n community s t r u c t u r e are products of n a t u r a l s e l e c t i o n ...." The c e n t r a l community c h a r a c t e r i s t i c here i s that of 'coadaptive a s s o c i a t i o n s or networks' and changes i n t h i s c h a r a c t e r i s t i c are r e f l e c t e d by changes i n a l l the o t h e r s . In t h i s context, the r e s u l t s from the present study may be used to f a s h i o n a q u a l i t a t i v e model f o r community e v o l u t i o n . U l t i m a t e l y , the model set f o r t h below i s concerned with i n t e r s p e c i f i c i n t e r a c t i o n and c o e x i s t e n c e , but the premise of the model c o n s i d e r s the community p r i m a r i l y as a c o l l e c t i o n of genotypes. T h i s i s i n keeping with the f a c t that the contact sampling method focuses on i n t e r s p e c i f i c i n t e r a c t i o n s from the p e r s p e c t i v e of the i n d i v i d u a l . In a d d i t i o n , Harper (1977b) has s t r e s s e d how c o n v e n t i o n a l d e s c r i p t i o n s of d i v e r s i t y based on the taxonomic s p e c i e s u n i t may not be e n t i r e l y adequate and that a major part of the f u n c t i o n a l d i v e r s i t y of a p l a n t community e x i s t s , at the i n t r a s p e c i f i c l e v e l . The proposed model t r e a t s g e n e t i c and s p e c i e s d i v e r s i t y as i n t e g r a l and interdependent components of community d i v e r s i t y r a t h e r than as separate phenomena. H y p o t h e t i c a l events are t r a c e d from the i n i t i a l ' c o l o n i z a t i o n ' (sowing of the pasture) to some f u t u r e s t a t e of the community which may be d e f i n e d a r b i t r a r i l y as a 'subclimax' maintained by g r a z i n g animals, although i t i s not intended that the model be c a s t as a d e s c r i p t i o n of s u c c e s s i o n i n the t r a d i t i o n a l sense of replacement of 'community types'. A t t e n t i o n i s focused on f i n e r - s c a l e events o c c u r r i n g i n l o c a l 97 neighbourhoods w i t h i n the pasture community. The model i s d i s p l a y e d i n F i g u r e 9 and d e s c r i b e d by the f o l l o w i n g stages: 1) When the pasture i s i n i t i a l l y ploughed and sown, a degree of both genetic and s p e c i e s d i v e r s i t y i s imparted on the community which together with the d i v e r s i t y represented i n the seed bank comprises the t o t a l ' i n i t i a l ' d i v e r s i t y , g e n e t i c and s p e c i e s , i n the community. 2) L o c a l d i s t u r b a n c e s (e.g. g r a z i n g , mole h i l l s , hoof marks), management p r a c t i c e s , v a r i a t i o n s i n c l i m a t e , emigration and immigration (gene f l o w ) , n a t u r a l death of p l a n t s or p l a n t p a r t s (and subsequent newly a v a i l a b l e space), and other chance events ( l o c a l d i s p e r s a l p a t t e r n s , mutations, genetic, d r i f t , e t c . ) represent the s t o c h a s t i c f a c t o r s r e s p o n s i b l e f o r a f f e c t i n g community change. Species and genetic d i v e r s i t y i n the community w i l l be a f f e c t e d (2A). These events provide o p p o r t u n i t i e s f o r d i f f e r e n t s p e c i e s and genotypes to combine and recombine with one another - to 'sample' t h e i r neighbours i n a. random and u n p r e d i c t a b l e manner. 3) The higher the d i v e r s i t y ( s p e c i e s and g e n e t i c ) , the more o p p o r t u n i t i e s there w i l l be f o r new 'random a s s o c i a t i o n ' , i . e . the g r e a t e r the p r o b a b i l i t y of d i f f e r e n t s p e c i e s and genotypes meeting randomly. Growth form w i l l a l s o i n f l u e n c e the number of o p p o r t u n i t i e s f o r s p e c i e s to 'sample' t h e i r neighbours. S t o l o n i f e r o u s s p e c i e s such as T r i f o l i u m repens (and to a l e s s e r extent rhizomatous s p e c i e s such as Poa compressa ) are able to 98 FIGURE 9. A q u a l i t a t i v e model f o r pasture community e v o l u t i o n . Steps i n the model are d e s c r i b e d i n the t e x t . 1 INITIAL COLONIZATION (when pasture i s ploughed and sown) ASSOCIATION 100 migrate w i t h i n the community, perhaps escape from i n h o s p i t a b l e neighbourhoods, and e v e n t u a l l y happen upon compatible ones. In t h i s context, the s t o l o n i f e r o u s s p e c i e s may be regarded as an i t i n e r a n t s p e c i e s and one which g r e a t l y i n f l u e n c e s the p r o p e n s i t y of i n i t i a l random a s s o c i a t i o n s . In the present study, T"\_ repens was i n v o l v e d i n more s i g n i f i c a n t a s s o c i a t i o n s (P<0.05) (most of which were 'temporary') d u r i n g the study p e r i o d than any other s p e c i e s (Table 11). T h i s i n f l u e n t i a l r o l e played by s t o l o n i f e r o u s s p e c i e s has been suggested by Turkington & Harper (1979c) and has been developed by Aarssen et a l . (1979). Depending on the c o m p a t i b i l i t y of the s p e c i e s i n v o l v e d i n these random a s s o c i a t i o n s , three d i f f e r e n t events may be p r e d i c t e d as f o l l o w s (Aarssen e_t a l . 1979): 3A) The a s s o c i a t i o n may be s i g n i f i c a n t but t r a n s i e n t . An a s s o c i a t i o n ( e i t h e r p o s i t i v e or negative) may break down (and perhaps reform) because of r e c u r r i n g d i s t u r b a n c e s . Species which respond to d i s t u r b a n c e i n s i m i l a r ways may tend to occur together and those which respond d i f f e r e n t l y may become mutually e x c l u s i v e . A s i g n i f i c a n t negative a s s o c i a t i o n may r e s u l t d u r i n g c o m p e t i t i v e e x c l u s i o n and may be temporary i f d i s t u r b a n c e h a l t s the e x c l u s i o n process and p e r i o d i c a l l y r e s t o r e s random a s s o c i a t i o n . T h i s i n t e r p r e t a t i o n a p p l i e s to the vast m a j o r i t y of a s s o c i a t i o n s l i s t e d i n Table 11. 3B) The a s s o c i a t i o n may be s i g n i f i c a n t only on a seasonal b a s i s . Seasonal c o m p a t i b i l i t y may r e s u l t i f two s p e c i e s have 101 asynchronous l i f e c y c l e s (temporal p a r t i t i o n i n g ) . Species which have some d i f f e r e n c e i n t h e i r e c o l o g i e s that permits t h e i r c o e x i s t e n c e , have " e c o l o g i c a l combining a b i l i t y " (Harper 1964, 1967). Mechanisms p e r m i t t i n g c o e x i s t e n c e c o n t r i b u t e to the maintenance of d i v e r s i t y i n the community (3B.1). Such seasonal d i f f e r e n c e s may be mediated by short term f l u c t u a t i o n s i n environment. In the present study, two seasonal a s s o c i a t i o n s were d e t e c t e d i n the 1977 past u r e ( F i g . 8d,i) and one i n the 1939 pasture ( F i g . 8 f ) . 3C) The a s s o c i a t i o n may be s i g n i f i c a n t and p e r s i s t e n t , i . e . a s t a b l e a s s o c i a t i o n . S t a b l e s i g n i f i c a n t a s s o c i a t i o n s are an important developmental f e a t u r e of the pastures i n the present study ( F i g s . 7 and 8). On a grand s c a l e , s p e c i e s are 'found together' because they are 'adapted' to a common type of h a b i t a t . The contact sampling method however r e v e a l s f i n e - s c a l e a s s o c i a t i o n at the c r i t i c a l l e v e l of i n d i v i d u a l e xperience. Such an a s s o c i a t i o n which i s s i g n i f i c a n t l y p o s i t i v e may a l s o be due to common s i t e requirements on a l o c a l s c a l e . If t h i s a s s o c i a t i o n i s s t a b l e however i t means that something i s pr e v e n t i n g c o m p e t i t i v e e x c l u s i o n . Species which c o e x i s t aire u s u a l l y thought to d i f f e r i n some way(s) i n which they e x p l o i t the environment (e.g see reviews by Grubb (1977) and Werner (1979)). E c o l o g i c a l combining a b i l i t y may take on s e v e r a l forms based on the notion that two s p e c i e s of c o n t r a s t i n g h a b i t with respect to some morphological or p h y s i o l o g i c a l c h a r a c t e r , w i l l together be able to e x p l o i t the t o t a l environment more e f f e c t i v e l y than both s p e c i e s growing alone. A resource-based 1 02 s e l e c t i v e advantage may be a f f o r d e d by such an a s s o c i a t i o n and t h e i r complementarity d i c t a t e s t hat they may be found growing i n pr o x i m i t y to one another more o f t e n than expected by chance. P o s i t i v e a s s o c i a t i o n may a l s o be encouraged by a b e n e f i c i a l i n t e r a c t i o n between s p e c i e s such as mutualism or commensal ism. S t a b l e (and s i g n i f i c a n t ) negative a s s o c i a t i o n may be a r e s u l t of some d i f f e r e n t i a t i o n on a h o r i z o n t a l s p a t i a l s c a l e with the consequence that the two s p e c i e s r a r e l y enter i n t o each other's immediate sphere of i n f l u e n c e . T h i s may confer an advantage i n terms of f i t n e s s i f i t serves to q u e l l t h r e a t s of c o m p e t i t i v e e x c l u s i o n . Harper (1967) has s t r e s s e d that an understanding of the c r i t i c a l d i f f e r e n c e s between s p e c i e s w i l l presumably provide the 'explanation' of s t a b l e d i v e r s i t y i n nature (3C.1). 4) Many a s s o c i a t i o n s w i l l remain random ( i . e . found together as o f t e n as expected by chance). Within each p o p u l a t i o n there w i l l be a range of ge n e t i c v a r i a b i l i t y r e f l e c t i n g a range of i n d i v i d u a l a b i l i t i e s to p e r s i s t i n a random a s s o c i a t i o n (with perhaps any one of many d i f f e r e n t s p e c i e s or genotypes). S e l e c t i o n a c t i n g on t h i s e x i s t i n g g e n e t i c v a r i a b i l i t y w i l l .oust from the community or from c e r t a i n neighbourhoods w i t h i n the community, those genotypes which do not ' f i t i n ' , i . e . are incompatible. A r e d u c t i o n i n ge n e t i c d i v e r s i t y may r e s u l t . Those genotypes which do ' f i t i n ' are those capable of r e s i d i n g i n neighbourhoods i n which immediate neighbours do not impose ove r b e a r i n g c o n s t r a i n t s on f i t n e s s - neighbourhoods where s u p e r i o r r i v a l s are not r e s i d e n t s . 1 03 5) The above s e l e c t i o n process leaves genotypes which are more compatible w i t h i n l o c a l neighbourhoods than the r e s i d e n t genotypes before s e l e c t i o n . T h i s may permit c o e x i s t e n c e and hence f u r t h e r c o n t r i b u t e s to the maintenance of s p e c i e s d i v e r s i t y . Having come f u l l c i r c l e in the model demonstrates i t s feedback p o t e n t i a l ; The steps 2 through 5 occur c o n t i n u o u s l y du r i n g the e v o l u t i o n of the community. The r e s u l t i s that some random a s s o c i a t i o n s remain random, but others may become s i g n i f i c a n t l y s t a b l e ( p o s i t i v e or negative) i f s e l e c t i o n i n v o l v e s niche d i f f e r e n t i a t i o n or beneficence (see 3C above). 6) As s e l e c t i o n r e s u l t s i n members of a p o p u l a t i o n adapted to d i f f e r e n t segments of the community, ' b i o t i c ecotypes' (Turesson 1922, Turkington & Harper 1979c) may e v o l v e . B i o t i c s p e c i a l i z a t i o n may evolve between s p e c i e s r e g a r d l e s s of whether they are found in a s s o c i a t i o n more often than expected by chance. Species which develop a more p e r s i s t e n t a s s o c i a t i o n d e v i a t i n g s i g n i f i c a n t l y from random e x p e c t a t i o n (e.g. F i g . 8) however a t t r a c t p a r t i c u l a r a t t e n t i o n and may e x h i b i t , the most r e c o g n i z a b l e b i o t i c ecotypes. Harper (1977b) has proposed that the p a t t e r n of a s s o c i a t i o n s i n o l d permanent p a s t u r e s i s l i k e l y to be the r e s u l t of a c l o s e e v o l u t i o n a r y c o a d a p t a t i o n i n v o l v i n g a l l the s p e c i e s . 7) As more and more b i o t i c s p e c i a l i z a t i o n i s evolved, the community i s c o n t i n u a l l y approaching the 'subclimax' which should t h e o r e t i c a l l y be c h a r a c t e r i z e d more and more by a ' f i x e d ' community matrix of unchanging i n t e r s p e c i f i c a s s o c i a t i o n s (some 1 04 of which w i l l be s i g n i f i c a n t l y s t a b l e ) - a stage which i s never f u l l y r e a l i z e d because of step 2. The net r e s u l t may be a decrease in g e n e t i c d i v e r s i t y ( i n terms of c o e x i s t i n g genotypes, not genes). A f t e r s e v e r a l i t e r a t i o n s , the model p r e d i c t s that the number of o p p o r t u n i t i e s f o r s p e c i e s and genotypes to combine and recombine i n a s s o c i a t i o n i s r e s t r i c t e d because the community i s t r a n s f i g u r e d from an e s s e n t i a l l y random and d i v e r s e c o l l e c t i o n of genotypes i n t o an assemblage of l a r g e l y s p e c i a l i z e d b i o t i c ecotypes which may a l s o be superimposed on a b i o t i c ecotypes. In o l d e r communities then, t h r e a t s of c o m p e t i t i v e e x c l u s i o n are l e s s severe and the response to d i s t u r b a n c e i n l o c a l neighbourhoods i s more p r e d i c t a b l e . Hence, there are fewer s i g n i f i c a n t temporary a s s o c i a t i o n s than i n younger communties. P l a c i n g the r e s u l t s i n t o a broad t h e o r e t i c a l context leads next to a c o n s i d e r a t i o n of p a r t i c u l a r s p e c i e s i n t e r a c t i o n s . Only in t h i s way can i n s i g h t be gained i n t o the mechanisms which may d r i v e the model. The .prouiinence of T r i f o l i u m repens i n s t a b l e p o s i t i v e a s s o c i a t i o n s and the prominence of s t a b l e negative a s s o c i a t i o n s between grasses i s noteworthy ( F i g . 8). Negative a s s o c i a t i o n may be expected amongst sp e c i e s such as grasses with s i m i l a r growth forms and hence perhaps s i m i l a r ways of e x p l o i t i n g the environment. Negative a s s o c i a t i o n may a l s o r e s u l t i f s p e c i e s have d i f f e r e n t s i t e requirements in a heterogeneous h a b i t a t . R e s u l t s from the s o i l analyses (Chapter 2) however showed very l i t t l e v a r i a t i o n w i t h i n f i e l d s and very l i t t l e c o r r e l a t i o n with s p e c i e s cover (Table 7). 105 Turkington & Harper (1979b) a t t r i b u t e a s t a b l e p o s i t i v e a s s o c i a t i o n between T\ repens and L o l ium perenne to an asynchrony of t h e i r growth c y c l e s . No c l e a r evidence of t h i s was found i n the present study and of p a r t i c u l a r i n t e r e s t i s the development of s t a b l e p o s i t i v e a s s o c i a t i o n between T\ repens and 5 d i f f e r e n t grasses ( F i g . 8 b , d , g , i , j ) . These p o s i t i v e a s s o c i a t i o n s i n v o l v i n g T\ repens may be r e l a t e d to s e l e c t i o n f o r some form of 'combining a b i l i t y ' and generated by a complex of f a c t o r s i n v o l v i n g the 'wandering' h a b i t of the s t o l o n i f e r o u s T. repens, 'seeking out' compatible neighbourhoods of grass genotypes, and a beneficence a f f o r d e d to the grasses by a c l o s e a s s o c i a t i o n with the n i t r o g e n - r i c h environment provided by the legume. Turkington & Harper (1979c) found that each of four d i f f e r e n t c l o n e s of T\_ repens e x h i b i t e d p r e c i s e b i o t i c s p e c i a l i z a t i o n to four r e s p e c t i v e l y d i f f e r e n t s p e c i es of neighbouring grass i n an o l d p a s t u r e . Very l i t t l e can be s a i d about the nature of s p e c i e s i n t e r a c t i o n s from contact sampling data alone. Contact sampling serves to d e t e c t departures' from random a s s o c i a t i o n of s p e c i e s : i and j - i t does not permit d e c i s i o n s to be made concerning'the r e l a t i v e n a t u r a l h o s t i l i t y or a f f i n i t y of i f o r j , versus j f o r i . I n t e r p r e t a t i o n s of p a t t e r n s i n terms of processes are only p o s s i b l e using c o n t r o l l e d experimental procedures. T h i s poses the q u e s t i o n as to what are the mechanisms producing a s s o c i a t i o n s and p e r m i t t i n g c o e x i s t e n c e i n these pastures? T h i s forms the l i n e of i n q u i r y i n subsequent c h a p t e r s . 106 CHAPTER 4 COMPETITIVE RELATIONS IN DIFFERENT AGED PASTURES A MIXTURE DIALLEL OF FIVE SPECIES 1 07 INTRODUCTION "The f a c t that organisms l i v i n g i n d i f f e r e n t p l a c e s are d i f f e r e n t i s easy to e x p l a i n by W a l l a c i a n f o r c e s . The q u e s t i o n of how so many s o r t s of organisms are able to p e r s i s t together i n the same 'place' i s much more d i f f i c u l t to answer, i s much more i n t e r e s t i n g ; i t demands b i o t i c i n t e r p r e t a t i o n and a Darwinian s o l u t i o n " (Harper 1977b). T r a d i t i o n a l s t u d i e s of i n t r a s p e c i f i c d i f f e r e n t i a t i o n w i t h i n p l a n t s p e c i e s have been dominated by a 'Wallacian' p e r s p e c t i v e c o n c e n t r a t i n g on l o c a l and l a r g e - s c a l e d i f f e r e n c e s amongst p o p u l a t i o n s i n r e l a t i o n to d i f f e r e n t q u a l i t i e s of a b i o t i c environmental f a c t o r s (e.g. Turesson 1922, Clausen Keck & Hiesey 1948, Kruckeberg 1951, J a i n & Bradshaw 1966, Snaydon 1970, 1971, Davies & Snaydon 1973, Teramura & S t r a i n 1979, Snaydon & Davies 1982). Other s t u d i e s have shown e c o t y p i c d i f f e r e n t i a t i o n between p o p u l a t i o n s i n r e l a t i o n to g r a z i n g (Kemp 1937), other forms, of d i s t u r b a n c e from human a c t i v i t i e s ( S o l b i r g & Simpson 1.974,. Warwick & Briggs . .1980), changes i n o v e r a l l h a b i t a t c o n d i t i o n s i n d i f f e r e n t stages of abandonment of a g r i c u l t u r a l f i e l d s (Hancock & Wilson 1976), and i n the c o n v e r s i o n of a woodland to a g r a s s l a n d (Lovett Doust 1981). Questions concerning s p e c i e s c o e x i s t e n c e however c e n t e r a t t e n t i o n on the r o l e of c o m p e t i t i v e i n t e r a c t i o n s as s e l e c t i o n a l f o r c e s and t h i s r e p r e s e n t s the f o c a l p o i n t of a 'Darwinian' p e r s p e c t i v e of i n t r a s p e c i f i c d i f f e r e n t i a t i o n . Only r e c e n t l y have e f f o r t s been made to search f o r and document i n t r a s p e c i f i c d i f f e r e n t i a t i o n a s s o c i a t e d with b i o t i c 108 s p e c i a l i z a t i o n i n response to c o m p e t i t i o n . Some s t u d i e s have re p o r t e d p o p u l a t i o n d i f f e r e n t i a t i o n i n a s i n g l e p l a n t s p e c i e s that appeared to be a s s o c i a t e d with the p a r t i c u l a r c o n s t i t u t i o n of the surrounding v e g e t a t i o n (Watson 1969, L i n h a r t 1974, Watson 1974 - i n Antonovics 1978). Other s t u d i e s have attempted to p i n p o i n t more p r e c i s e l y the b i o t i c elements i n v o l v e d . Remison and Snaydon ( i n Snaydon 1978) demonstrated e c o t y p i c d i f f e r e n t i a t i o n i n p o p u l a t i o n s of Anthoxanthum odoratum, D a c t y l i s glomerata , Holcus l a n a t u s and Lolium perenne ; p a i r s of p o p u l a t i o n s c o l l e c t e d from the same s i t e had a higher combined y i e l d when grown i n experiment together than d i d p o p u l a t i o n s c o l l e c t e d from d i f f e r e n t s i t e s . Turkington & Harper (1979c) demonstrated f i n e - s c a l e p o p u l a t i o n s p e c i a l i z a t i o n i n T r i f o l i u m repens in response to d i f f e r e n t s p e c i f i e d neighbouring s p e c i e s w i t h i n a s i n g l e pasture l e s s than 1 ha i n s i z e . Martin & Harding (1981) a l s o showed evidence f o r b i o t i c s p e c i a l i z a t i o n i n sympatric versus a l l o p a t r i c p o p u l a t i o n s of Erodium o b t u s i p l i c a t u m and E^ c i c u t a r i u m . The i m p l i c a t i o n of these f i n d i n g s i s that such d i f f e r e n t i a t i o n i s a consequence of gen e t i c changes r e f l e c t i n g l o c a l adjustment ( s p e c i a l i z a t i o n ) to the b i o t i c environment. Moreover, such i n t r a s p e c i f i c d i f f e r e n t i a t i o n may e x i s t not only between p o p u l a t i o n s , but a l s o w i t h i n a s i n g l e p o p u l a t i o n (e.g. Turkington & Harper 1979c). Some s t u d i e s have i n d i c a t e d that a s p e c i f i c p a i r of genotypes may be s p e c i a l i z e d to each other i n r e l a t i o n to other genotypes of the same two s p e c i e s ( A l l a r d & Adams 1969, Remison & Snaydon i n Snaydon 1978, Joy & L a i t i n e n 1980). In the context of m u l t i s p e c i e s i n t e r a c t i o n s , t h i s underscores the modus operandi 109 of " b i o l o g i c a l accommodation" (Saunders 1968, McNaughton & Wolf 1979) in communities - the nucleus of an e v o l u t i o n a r y p e r s p e c t i v e of s p e c i e s c o e x i s t e n c e . The c r u c i a l q u e s t i o n l e f t unanswered by p r e v i o u s s t u d i e s of b i o t i c s p e c i a l i z a t i o n concerns the p r e c i s e s e l e c t i o n mechansism(s) by which s p e c i e s a d j u s t to t h e i r environment of neighbours, r e s u l t i n g i n more compatible behaviour i n t h e i r presence. Previous i n t e r p r e t a t i o n s of b i o t i c s p e c i a l i z a t i o n concern ' e c o l o g i c a l combining a b i l i t y ' (Harper 1964, 1977b) implying an evolved niche displacement. T h i s c o n f e r s some measure of escape from c o m p e t i t i o n with the r e s u l t that s p e c i e s y i e l d higher together in mixture. T h i s i n t e r p r e t a t i o n seems to apply f o r example to the data of Remison & Snaydon ( i n Snaydon 1978). B i o t i c s p e c i a l i z a t i o n i n the pasture p o p u l a t i o n of T r i f o l i u m repens d e s c r i b e d above has been i n t e r p r e t e d as evidence f o r l o c a l 'coadaptation' between white c l o v e r and grasses i n v o l v i n g a process of s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y (Harper 1977b). There was no attempt however in the study t o : t e s t f o r r e c i p r o c a l s p e c i a l i z a t i o n i n the grasses, so i t i s not c e r t a i n whether t h i s i s evidence f o r c o a d a p t a t i o n or simply one-way ad a p t a t i o n ( s p e c i a l i z a t i o n ) by the c l o v e r . Furthermore, without i n f o r m a t i o n about the r e c i p r o c a l e f f e c t s of competitors, i t i s not p o s s i b l e to d i s t i n g u i s h whether such l o c a l s p e c i a l i z a t i o n i s a consequence of s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y (niche s h i f t ) , or s e l e c t i o n which improves r e l a t i v e f i t n e s s through an improvement i n the r e l a t i v e c a p a c i t y to reduce the a v a i l a b i l i t y of resources to a competitor. 1 10 The best r e f l e c t i o n of the way(s) in which neighbours respond r e c i p r o c a l l y to each other must come from i n v e s t i g a t i o n s i n v o l v i n g i n d i v i d u a l s which a c t u a l l y i n t e r a c t i n nature, yet no former attempt at t h i s has been made. I n s i g h t i n t o the s e l e c t i o n mechanisms producing b i o t i c s p e c i a l i z a t i o n i s only p o s s i b l e through studying how the r e c i p r o c a l responses between n a t u r a l neighbours i n a community change through time or i n d i f f e r e n t aged communities with an e s t a b l i s h e d ontogenetic r e l a t i o n s h i p . Furthermore, no previous e f f o r t s have been made to e l u c i d a t e the process of b i o l o g i c a l accommodation in a community-wide framework. T h i s r e q u i r e s a study of the changes in r e c i p r o c a l responses between n a t u r a l neighbours f o r s e v e r a l of the most dominant s p e c i e s i n a community. These are the o b j e c t i v e s of the present i n v e s t i g a t i o n . One approach to understanding the r e l a t i o n s h i p s amongst the members of a community i s to i s o l a t e the members and combine them in a l l p o s s i b l e p a i r s . In t h i s experimental design, c a l l e d a ' d i a l l e l a n a l y s i s ' , a number of s p e c i e s are grown in a l l p o s s i b l e combinations.of p a i r s and t h e i r y i e l d s as mixtures are c o n t r a s t e d with t h e i r y i e l d s as pure stands (Norrington-Davies 1967, 1968, Trenbath 1975, 1978). Both the y i e l d d i f f e r e n t i a l between the two component s p e c i e s and t h e i r t o t a l y i e l d provide i n f o r m a t i o n f o r the a n a l y s i s of r e c i p r o c a l e f f e c t s of c o m p e t i t i o n . I t i s a l s o i n t e r e s t i n g to ask whether a p a r t i c u l a r p o p u l a t i o n e x h i b i t s a s t a b l e or s e n s i t i v e behaviour i n response to d i f f e r e n t a s s o c i a t e s . Such i n f o r m a t i o n on the ' s o c i o l o g i c a l homeostasis' of mixtures can be obtained from the a n a l y s i s of mixture d i a l l e l s (Jaquard & Caputa 1970, Rousvoal & G a l l a i s 111 1973). The above i n f o r m a t i o n i s sought i n the present study using a v a r i a t i o n of the d i a l l e l set-up i n v o l v i n g 5 of the most dominant s p e c i e s i n the d i f f e r e n t aged pastures under study. The p r i n c i p a l aim i s to t e s t f o r the occurrence of developing b i o l o g i c a l accommodation d u r i n g pasture community e v o l u t i o n , and to c h a r a c t e r i z e the p o s s i b l e mechanisms that may be i n v o l v e d . METHODS A l l p o s s i b l e 2-species p a i r s of n a t u r a l neighbouring genets were c o l l e c t e d from each of the three d i f f e r e n t aged pastures and c l o n e d , as o u t l i n e d i n Chapter 2, f o r the f o l l o w i n g f i v e s p e c i e s : D a c t y l i s glomerata , Holcus lanatus , Lolium perenne , Poa compressa and T r i f o l i u m repens . In the f i r s t week of May 1980, 25 ramets of each genet were p l a n t e d with 25 ramets of i t s n a t u r a l neighbouring genet i n 25 cm x 25 cm f i e l d p l o t s in the arrangement shown in F i g u r e 10. Since each genet type was d e f i n e d by a s p e c i f i c n a t u r a l neighbouring s p e c i e s , there was no b a s i s f o r d e c i d i n g upon the genotypic composition f o r •monoculture plots,* these -were t h e r e f o r e omitted.- Ramets of each of the above s p e c i e s were a l s o obtained from a stock source of p l a n t s d e r i v e d from a commercial seed supply (Richardson's Seed Co., Burnaby, B.C.) and p l a n t e d i n a l l p o s s i b l e s p e c i e s p a i r s of 25 ramets each, as above. T h i s represented ' n a t u r a l neighbours' from a pasture of age 'zero'. A l l mixtures were r e p l i c a t e d 3 times. P l o t s were set up 1.5 m apart and arranged randomly in the f i e l d at the experimental f i e l d s t a t i o n , U n i v e r s i t y of B r i t i s h Columbia. Successive h a r v e s t s were taken by p l a c i n g a square 1 1 2 FIGURE 10. P l a n t i n g arrangement in the mixture d i a l l e l showing p o s i t i o n s f o r 25 ramets of each of two s p e c i e s . The dashed l i n e s i n d i c a t e the s i z e and p o s i t i o n i n g of the h a r v e s t i n g frame p l a c e d over the p l o t w i t h i n which the v e g e t a t i o n was c l i p p e d and c o l l e c t e d at each h a r v e s t . 113 5 cm < > • • • • • o f" o o o o~[ j • • • • j • o I o o o o I • • • • I • o I o o o o i I • • • • I • o J o o o o I • • • • I • I I o o o o o 1 1 4 h a r v e s t i n g frame over each p l o t ( F i g . 10) and c l i p p i n g the v e g e t a t i o n i n s i d e the frame to approximately 3 cm. The c l i p p i n g s were c o l l e c t e d , separated a c c o r d i n g to s p e c i e s , d r i e d and weighed. The f i r s t h a rvest was taken 10 weeks a f t e r p l a n t i n g the ramets. Four more h a r v e s t s were taken at approximately 3-4 week i n t e r v a l s u n t i l November. N e g l i g i b l e growth occ u r r e d over winter and h a r v e s t i n g resumed in the f i r s t week of A p r i l 1981 and continued at 3-4 week i n t e r v a l s u n t i l mid-July 1981, one year a f t e r the f i r s t h a r v e s t . In the f i n a l h a r v e s t , the t o t a l above ground p a r t s w i t h i n each p l o t were c o l l e c t e d , d r i e d and weighed and the t o t a l cummulative dry weights f o r each of the two components over the one year p e r i o d were determined. A n a l y s i s of v a r i a n c e used i n data a n a l y s i s i n v o l v e d a MIDAS program (Fox & Guire 1976). RESULTS It i s f i r s t i n t e r e s t i n g to compare the mean performance and v a r i a b i l i t y i n performance of each s p e c i e s with t h e i r d i f f e r e n t a s s o c i a t e s (Table 13) . The var i a b i l i t y of 'performance . for. a s p e c i e s from a given f i e l d age i s measured by the c o e f f i c i e n t - of v a r i a t i o n i n Table 13 and r e f l e c t s the s e n s i t i v i t y of that s p e c i e s to the nature of i t s a s s o c i a t e s i n i t s community. Holcus l a n a t u s g e n e r a l l y showed the lowest s e n s i t i v i t y to the nature of i t s a s s o c i a t e s (except i n the 21 year pasture where i t was second l o w e s t ) . T r i f o l i u m repens g e n e r a l l y showed the h i g h e s t s e n s i t i v i t y to the nature of i t s a s s o c i a t e s (except i n the 0 year pasture where i t was second h i g h e s t ) . S t a t i s t i c a l a n a l y s i s showed that f o r each s p e c i e s (and f o r a l l s p e c i e s 1 15 TABLE 13. 5 x 5 matrices of p e r - s p e c i e s y i e l d s (g) (means of 3 r e p l i c a t e s ) from the mixture d i a l l e l i n v o l v i n g 5 s p e c i e s p l a n t e d as ramets. In a) a l l ramets were d e r i v e d from a commercial seed source (pasture age ' z e r o ' ) . In b), c) and d) a l l ramets were cl o n e s of 2 n a t u r a l neighbouring genets c o l l e c t e d from the 2, 21 and 40 year o l d pastures r e s p e c t i v e l y . P r o b a b i l i t y l e v e l s are given from the a n a l y s i s of v a r i a n c e of mean y i e l d s , and from an F - t e s t f o r the e q u a l i t y of v a r i a n c e s f o r each s p e c i e s with a l l a s s o c i a t e s from a given' f i e l d age. A t t a i n e d P l e v e l s when a l l s p e c i e s from a given f i e l d age are averaged (*) are a l s o shown. C.V. = c o e f f i c i e n t of v a r i a t i o n . D.g. = D a c t y l i s glomerata ; H.1. = Holcus lanatus ; L.p. = Lolium perenne ; P.c. = Poa compressa ; T . r . = T r i f o l i u m repens . 1 1 6 ASSOCIATES PRODUCERS D.g. H . l . L.p. P.c. T . r . Mean (*) D.g. 121.6 113.0 2.2 35.8 H . l . 116.0 - 101.3 1 .0 21.8 a) L.p. 1 09.4 116.7 - 1 .6 46.5 P.c. 257.8 1 79. 1 181.5 - 91 .6 T. r . 181.3 166.2 180.0 17.7 — mean 1 66. 1 145.9 1 44.0 5.6 48.9 102.1 s t . dev. 69.2 31.4 42.8 8.1 30.2 36.3 C.V. 0.42 0.22 0.30 1 .45 0.62 D.g. •- 76.3 106.7 9.7 1 .7 H . l . 223. 1 - 35.7 16.4 29.5 b) L.p. 179.9 212.9 - 11.4. 70.8 P.c . 369.9 143.6 187.6 - 81 .2 T. r . 448.3 144.7 108.6 26.4 — mean 305.3 1 44.4 109.7 16.0 45.8 124.2 s t . dev. 125.3 55.8 62. 1 7.5 36.9 57.5 C.V. 0.41 0.39 0.57 0.47 0.81 D.g. - 84.6 70.3 12.3 3.3 •H.l. 226.6 - 58.5 7.3 2.2 c) L.p. 273.3 1 82.9 - 5.2 11.4 P.c . 270.6 242.3 180.2 - 59.4 T.r. 419.6 225.2 1 62 . 4 24. 1 — mean 297.5 183.8 117.9 12.2 19.1 1 26. 1 s t . dev 84.2 70.7 62.3 8.5 27.2 50.6 C.V. 0.28 0.38 0.53 0.70 1 .42 . . D.g. .- -• 150.5 .33. 1 ., .10.5. . • 5. 4 H . l . ' 221 .4 - 73.3 13.8 • 16.2 d) L.p. 291 .4 128.5 - • 11.2. 72.0 P.c . 346.9 211.3 163.0 - 54.3 T.r. 408.6 172.5 61 .3 36.6 — mean 317.1 165.7 82.7 18.0 37.0 124.1 s t . dev. 79.8 35.5 56. 1 12.5 31.4 43.0 C.V. 0.25 0.21 0.68 0.69 0.85 P ( a n a l y s i s of v a r i a n c e ) 0.1230 0.6662 0.5167 0.3007 0.5596 0.9823 P ( e q u a l i t y of v a r i a n c e s ) 0.7743 0.5271 0.9313 0.8254 0.9677 0.7431 117 averaged), there was a remarkable c o n s i s t e n c y amongst p l a n t s from the d i f f e r e n t aged pastures i n t h e i r mean y i e l d s and v a r i a n c e s with a l l a s s o c i a t e s . A t t e n t i o n now turns to the d i f f e r e n c e s i n r e c i p r o c a l responses between p a r t i c u l a r neighbouring s p e c i e s from d i f f e r e n t aged p a s t u r e s . An index of 'combining a b i l i t y ' was de v i s e d as a r e f l e c t i o n of the r e c i p r o c i t y of c o m p e t i t i v e e f f e c t s between neighbours. Combining a b i l i t y i s c a l c u l a t e d as CA = Y/Y' where Y and Y' are the recorded dry weight y i e l d s (g) of the lower and higher y i e l d i n g components of a mixture r e s p e c t i v e l y . CA i n d i c e s have s t r i c t l y r e l a t i v e value i n comparing combinations of a given neighbouring genet p a i r type ( i . e . of the same two sp e c i e s ) from d i f f e r e n t aged p a s t u r e s . Values of CA c l o s e r to 1.0 i n d i c a t e that the two components are more balanced in t h e i r y i e l d s i n mixtures than are combinations having a value c l o s e r to zero. Changes i n the nature of r e c i p r o c a l c o m p e t i t i v e e f f e c t s f o r a .given neighbouring, s p e c i e s p a i r are encompassed by two. parameters: 1) changes i n t o t a l y i e l d of the combination, and 2) changes in the r e l a t i v e c o n t r i b u t i o n to t o t a l y i e l d from the two components as r e f l e c t e d by the CA index. A comparison of i n d i v i d u a l y i e l d s , t o t a l y i e l d s and CA indexes f o r each genet p a i r type from the d i f f e r e n t aged pastures i s presented i n F i g u r e 11. Trends i n the data with i n c r e a s i n g pasture age are summarized i n Table 14. 118 FIGURE 11. Comparisons of i n d i v i d u a l component y i e l d s , t o t a l y i e l d s and combining a b i l i t y i n d i c e s (CA) f o r each genet p a i r type c o l l e c t e d from d i f f e r e n t aged pastures (0, 2, 21 and 40 years) and grown in competition i n a mixture d i a l l e l . In each case histograms represent a mean of 3 r e p l i c a t e s . In comparing the four f i e l d ages (with respect to component y i e l d , t o t a l y i e l d s or CA i n d i c e s ) , those which do not share a common code l e t t e r (a, b, c or d) are s i g n i f i c a n t l y d i f f e r e n t at the P<0.05 l e v e l based on Sc h e f f e ' s m u l t i p l e comparison t e s t . F - t e s t s f o r the homogeneity of v a r i a n c e s were not r e j e c t e d and a l l analyses were performed on untransformed data. YIELD (g) *-» Ln O en O o o 1 . 1 1 to Go -fc* o o O o o o O o 1 1 • 1 cr cr n 0) cr\ to O TOTAL YIELD CA CO to o o o o o o o o • • • • o o o o o to 4̂- oo L_ 1 1 1 1 1 1 1... o o 2 tr) jo > > T3 CD JO m as z en YIELD (g) en O O Cn O O en o cr to o Cn o o O o 1 1 1 TOTAL YIELD CA to to 4̂ o O o 2 cn jo > > o o L_ Co o o to o o _L_ o o _L_ cr o o o o • • • • to 1 4̂ oo 1 to > > H G 00 cr to 4̂ O YIELD (g) Oo o to o o _l l_ I—* to Oo o o O o o o o o o o • 1 1 I 1 TOTAL YIELD CA o o OO o o to o o _l_ o o _l_ o o I—I _L_ o to JL_ o _l_ tO O _l to cr cr a to o o f o jo > > JO W m YIELD (g) ""TJ o ui o o __J I 1 L to O o TOTAL YIELD oo o o CA to to O a t-o 2 tn > > J> Oo o o o o _J l _ to o o _1_ o o o to _l_ o _ l _ n o 2 . po PI cn en > to o YIELD (g) to O _L_ I to o o o o o o 1 1 1 I O to cr cr a o a 4*> O TOTAL YIELD CA o o to I-1 l - 1 • • O Ln O Ln O >— O O O O O O 1 1 1 1 cr cr ac ab be to to 4*. o YIELD (g) ?0 PI GO GO > o o VJ O to L/1 P" > Z > H C CD TOTAL YIELD o o to to o o O o o o 1 1 1 o o o o O to o o to to 4*. o CD 1 = > z > H GO CA o o o O • • • • to 1 1 OO 1 P3 CD 73 tn z z PI to 4̂ o YIELD (g) o _1_ _L_ Oi O I— tO O Ui o 0 o o 1 » L_ to o TOTAL YIELD tO O On O Oi o O o o 1 1 o to _1_ CA o _ L _ o o OO to o YIELD (g) m m z z m o o ?0 m co CO > o _JL_ to o _1_ o JL_ Ol o _JL_ to to o Oi o Oi o o o 1 O 1 0) cr to to o TOTAL YIELD CA to 1—1 o On o On o 1 O 1 o O 1 o >- On O On _L_ 8 cr on > z > H CO tn z CO to to to to cr 0) cr o YIELD (g) o o O l o o _L_ to o O o B> cr B) cr to TOTAL YIELD o o _1_ O l o _1_ fll cr B> cr CA o o o o • • • • to 1 1 1 GO 1 o o o 2 73 in GO GO > 73 m z GO B) cr o YIELD (g) o o O l o O l o TOTAL YIELD to o O l o o o o 1 1 1 cr o B) cr CA o to O CD ?D m z z en to to o O l o o o 1 o 1 O l o _1_ o o o o •• • • • to 00 1 1 1 1 73 m -o cn z GO fl) cr to Bi cr to 1 24 TABLE 14. A summary of s i g n i f i c a n t (P<0.05) trends i n combining a b i l i t y indexes (CA) and t o t a l y i e l d s from the mixture d i a l l e l f o r d i f f e r e n t genet p a i r types as the age of the pasture from which they were c o l l e c t e d i n c r e a s e s ( F i g . 11). An upward arrow i n d i c a t e s t h a t . t h e value i n c r e a s e s c o n s i s t e n t l y with i n c r e a s i n g pasture age, and a downward arrow i n d i c a t e s a c o n s i s t e n t decrease with i n c r e a s i n g pasture age. Both arrows together i n d i c a t e a f l u c t u a t i n g t r e n d . A dash (-) i n d i c a t e s no s i g n i f i c a n t change with i n c r e a s i n g pasture age. An a s t e r i s k (*) means that the i n d i c a t e d t rend occurs i f the '0-year' combination i s excluded. CA TOTAL YIELD DACTYLIS/ DACTYLIS/ DACTYLIS/ DACTYLIS/ HOLCUS LOLIUM POA TRIFOLIUM T- 4, t J, t 4, - t HOLCUS/ HOLCUS/ HOLCUS/ LOLIUM/ LOLIUM/ POA/ LOLIUM POA TRIFOLIUM POA TRIFOLIUM. TRIFOLIUM t* n n n n t - n % - ± 126 DISCUSSION Recent s t u d i e s have demonstrated d i f f e r e n c e s i n niche and co m p e t i t i v e r e l a t i o n s h i p s amongst s p e c i e s from communities of d i f f e r e n t s u c c e s s i o n a l m aturity ( P a r r i s h & Bazzaz 1976, 1979, 1982a, 1982b). These s t u d i e s have shown that s p e c i e s from ' l a t e - s u c c e s s i o n ' communities are more d i f f e r e n t i a t e d with respect to c e r t a i n dimensions of niche than are s p e c i e s from ' e a r l y - s u c c e s s i o n ' communities and t h i s i s proposed to be an evolved product of s e l e c t i o n a l f o r c e s from c o m p e t i t i o n . In a l l of these s t u d i e s , comparisons have been made between communities which d i f f e r in s p e c i e s composition and hence, the co m p e t i t i v e r e l a t i o n s of e n t i r e l y d i f f e r e n t groups of s p e c i e s have been compared i n making the above c o n c l u s i o n s . There i s a l s o not a strong b a s i s f o r c o n s i d e r i n g that the d i f f e r e n t communities being compared in these s t u d i e s even belong to the same s u c c e s s i o n a l sequence. T h i s presents c o n s i d e r a b l e u n c e r t a i n t y in making any i n f e r e n c e s about the importance of past c o m p e t i t i o n i n the o r g a n i z a t i o n of a community over e v o l u t i o n a r y time (see a l s o C o n n e l l 1 980). ' There have a p p a r e n t l y been no former attempts to answer the qu e s t i o n of how the c o m p e t i t i v e r e l a t i o n s h i p between two part i c u l a r s p e c i e s changes through time i n a pa r t i c u l a r community; Nor have p r e v i o u s s t u d i e s been based on i n t e r a c t i o n s between i n d i v i d u a l s which were known to be immediate neighbours in nature. The present study i s an attempt to address these problems u s i n g a s e r i e s of d i f f e r e n t aged pastures which represent a reasonable approximation of d i f f e r e n t c o e x i s t e n t stages i n the same developmental sequence based on known 1 27 management h i s t o r y as w e l l as e m p i r i c a l data (Chapters 2 and 3). Using t h i s approach, i n t e r p r e t a t i o n s concerning the r o l e of competition as a s e l e c t i o n a l f o r c e i n communities can be made with more confidence than i n comparisons of communities of widely d i f f e r e n t s p e c i e s composition having no e s t a b l i s h e d ontogenetic r e l a t i o n s h i p . The r e s u l t s showed that the o v e r a l l s e n s i t i v i t y ( v a r i a b i l i t y of response) to d i f f e r e n t neighbours f o r any given s p e c i e s w i t h i n a pasture d i d not change s i g n i f i c a n t l y with pasture age (Table 13). N a t u r a l neighbouring genets of a p a r t i c u l a r p a i r of s p e c i e s however, showed c l e a r d i f f e r e n c e s i n t h e i r r e l a t i v e reponses to one another depending on t h e i r pasture age of o r i g i n ( F i g . 11). The r e p o r t e d data lend i n s i g h t i n t o the c h a r a c t e r i z a t i o n of the development of b i o l o g i c a l accommodation in systems of c o m p e t i t i o n . In order to focus a t t e n t i o n on mechanisms, based on the o p e r a t i o n of n a t u r a l s e l e c t i o n in r e l a t i o n to the a t t r i b u t e s of organisms which c o n f e r f i t n e s s , the development of b i o l o g i c a l accommodation in a community may be t r a n s l a t e d as s e l e c t i o n f o r 'combining a b i l i t y ' i n populations.. Species which have combining a b i l i t y have some means of p e r s i s t i n g i n i n t e r a c t i o n with one another' d e f i n e d by t h e i r r e s p e c t i v e b i o l o g i c a l a t t r i b u t e s . An e x p l a n a t i o n of c o m p e t i t i v e c o e x i s t e n c e t h e r e f o r e r e s i d e s i n the assumption that a consequence of n a t u r a l s e l e c t i o n i s i n c r e a s e d (or s t a b i l i z e d ) combining a b i l i t y . Conversely, c o m p e t i t i v e e x c l u s i o n would be r e f l e c t e d by a d e c r e a s i n g combining a b i l i t y and may a l s o be a consequence of n a t u r a l s e l e c t i o n . Three f e a t u r e s of the data may be r e c o g n i z e d from examining the r e l a t i v e changes in the 1 28 t o t a l and r e s p e c t i v e component y i e l d s of genet combinations from o l d e r pastures ( i . e . ' a f t e r s e l e c t i o n ' ) ( F i g . 11, Table 14): 1) A process of c o m p e t i t i v e e x c l u s i o n i s r e f l e c t e d by a d e c l i n e i n combining a b i l i t y . In two s p e c i e s p a i r s , D a c t y l i s / L o l i u m and D a c t y l i s / T r i f o l i u m , combining a b i l i t y decreases with i n c r e a s i n g pasture age. T h i s suggests that the D a c t y l i s p o p u l a t i o n has become r e l a t i v e l y more co m p e t i t i v e a g a i n s t both Lolium and T r i f o l i u m i n o l d e r p a s t u r e s as a r e s u l t of s e l e c t i o n . In t h i s case, a v a i l a b l e resource u n i t s are p a r t i t i o n e d l e s s evenly between two p o p u l a t i o n s a f t e r s e l e c t i o n , i n c r e a s i n g the p r o b a b i l i t y that one w i l l exclude the other. 2) S e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y would be r e f l e c t e d by an i n c r e a s e i n t o t a l y i e l d a f t e r s e l e c t i o n (accompanied by a s t a b l e or i n c r e a s i n g combining a b i l i t y ) . T h i s i m p l i e s a displacement in behaviour with the r e s u l t that each (or one) p o p u l a t i o n makes l e s s demands on the resources needed by the other than before s e l e c t i o n . I t i s remarkable that only one s p e c i e s combination, D a c t y l i s / H o l c u s i l l u s t r a t e d t h i s t r e n d . 3) The most notable f e a t u r e of the data i s that a change in y i e l d f o r one component (e.g. an increase) i s u s u a l l y accompanied by an o p p o s i t e change in y i e l d ( i . e . a decrease) f o r the other component ( l e f t - h a n d bar graphs in F i g . 11). The consequence of t h i s i s that combining a b i l i t y changes a c c o r d i n g l y , but that t o t a l y i e l d r a r e l y shows s i g n i f i c a n t changes (ri g h t - h a n d bar graphs in F i g . 11). In two s p e c i e s p a i r s , Holcus/Lolium and P o a / T r i f o l i u m , while combining a b i l i t y (CA) a l s o i n c r e a s e s with i n c r e a s i n g pasture age, t o t a l y i e l d s i n mixture were not s i g n i f i c a n t l y d i f f e r e n t with i n c r e a s i n g pasture 1 29 age. T h i s r e s u l t does not i n d i c a t e that c o mpetition has been r e l a x e d due to niche divergence; r a t h e r , the trends suggest that s e l e c t i o n has improved the r e l a t i v e a b i l i t y of the i n f e r i o r component to reduce the a v a i l a b i l i t y of resources to the s u p e r i o r component , thus r e s u l t i n g i n a more balanced p a r t i t i o n i n g of resource u n i t s from a common supply on which they both make demands. The t r e n d i s s i m i l a r i n the D a c t y l i s / P o a combination but combining a b i l i t y d e c l i n e s between the 1958 and 1939 p a s t u r e s . The remaining combinations show no d i r e c t i o n a l trends ( F i g . 11 f , g , h , i ) . N either a process of c o m p e t i t i v e e x c l u s i o n nor a process of e c o l o g i c a l combining a b i l i t y i s i n d i c a t e d . As these are t r a d i t i o n a l l y the expected a l t e r n a t i v e outcomes of s e l e c t i o n p ressure from c o m p e t i t i o n , i t may suggest that these c o m p e t i t i v e i n t e r a c t i o n s are not important f o r c e s of s e l e c t i o n here. However, an i n t e r e s t i n g f e a t u r e of these combinations suggests an a l t e r n a t i v e e x p l a n a t i o n - an i n c r e a s e i n the y i e l d of one component i s g e n e r a l l y accompanied by a decrease i n the other and v i c e v e r s a . In the two combinations, i n v o l v i n g Holcus the t o t a l y i e l d a l s o f l u c t u a t e s but i n negative r e l a t i o n s h i p to the f l u c t u a t i o n i n combining a b i l i t y ( F i g . 11 f & g ) . In the two combinations i n v o l v i n g Lolium, combining a b i l i t y f l u c t u a t e s but the h i g h e s t value occurs i n the combination from the o l d e s t p a s t u r e . A l s o , i n the Lolium/Poa combination, t o t a l y i e l d i s the same with i n c r e a s i n g pasture age ( F i g . 11 h) and i n the Lo1ium/Trifo1iurn combination, t o t a l y i e l d decreases ( F i g . 11 i ) . Although none of these r e s u l t s are c o n s i s t e n t with an i n t e r p r e t a t i o n of s e l e c t i o n l e a d i n g to e i t h e r c o m p e t i t i v e 1 30 e x c l u s i o n or niche d i f f e r e n t i a t i o n , they are c o n s i s t e n t with a s e l e c t i o n process i n which r e l a t i v e c o m p e t i t i v e a b i l i t i e s f o r common niche requirements are c o n t i n u o u s l y a d j u s t e d so that over time a balance i s maintained. These data suggest that i n t r a s p e c i f i c d i f f e r e n t i a t i o n a s s o c i a t e d with b i o t i c s p e c i a l i z a t i o n i n response to competition may be r e l a t e d to three a l t e r n a t i v e consequences of n a t u r a l s e l e c t i o n : a) i n c r e a s e d combining a b i l i t y r e s u l t i n g from niche d i f f e r e n t i a t i o n and r e l e a s e from resource c o m p e t i t i o n ; b) i n c r e a s e d combining a b i l i t y r e s u l t i n g from a 'balancing' of c o m p e t i t i v e a b i l i t i e s for c o n t e s t e d r e s o u r c e s ; c) reduced combining a b i l i t y r e s u l t i n g from asymmetic improvement (or 'unbalancing') of c o m p e t i t i v e a b i l i t i e s . N o t i c e that i n both a) and b) i t i s assumed that s e l e c t i o n reduces the d i f f e r e n t i a l in the performance of the two components of the mixture. The i m p l i c a t i o n of t h i s i s that the two p o p u l a t i o n s have a g r e a t e r p r o b a b i l i t y of c o e x i s t i n g than before s e l e c t i o n , , i . e . i n both cases the two p o p u l a t i o n s have higher 'combining a b i l i t y ' . Only case a) however, where t o t a l y i e l d i n c r e a s e s , permits an i n t e r p r e t a t i o n of ' e c o l o g i c a l ' combining a b i l i t y as p r e v i o u s l y d e f i n e d (Harper 1977b). In c) there has been s e l e c t i o n f o r b i o t i c s p e c i a l i z a t i o n i n the s u p e r i o r competitor, s e l e c t i o n which reduces combining a b i l i t y . S e l e c t i o n o p e r a t i n g i n a) and b) then r e s u l t i n b i o l o g i c a l accommodation and c o e x i s t e n c e , while s e l e c t i o n o p e r a t i n g i n c) r e s u l t s i n c o m p e t i t i v e e x c l u s i o n . In c o n c l u s i o n , the changes d i s c o v e r e d i n the combining 131 a b i l i t y and t o t a l y i e l d s of mixtures of a given p a i r of n a t u r a l l y neighbouring s p e c i e s from d i f f e r e n t aged p a s t u r e s , suggest that competition i s an important f o r c e of n a t u r a l s e l e c t i o n i n t h i s pasture system. I t i s evident that a f a i l u r e to study the r e c i p r o c a l e f f e c t s of competitors on one another does not permit a c l e a r i n t e r p r e t a t i o n of the mechanisms of b i o t i c s p e c i a l i z a t i o n and combining a b i l i t y of s p e c i e s . For example, the b i o t i c s p e c i a l i z a t i o n i n T r i f o l i u m repens r e p o r t e d by Turkington & Harper (1979c) c o u l d be a s s o c i a t e d with any of the above 3 consequences of n a t u r a l s e l e c t i o n (a, b or c ) . Very l i t t l e evidence i n the present study (only one s p e c i e s combination) ( F i g . 11a) was found f o r the commonly assumed no t i o n that s e l e c t i o n p r e s s u r e s from competition u s u a l l y r e s u l t in the e v o l u t i o n of e c o l o g i c a l combining a b i l i t y (niche d i f f e r e n t i a t i o n ) i n the component s p e c i e s of a community. F i v e out of the ten s p e c i e s combinations s t u d i e d ( F i g . 1 1 c , e, h, i , j ) e x h i b i t e d i n c r e a s e d combining a b i l i t y with i n c r e a s i n g pasture age nonetheless, but i t was of a form i n t e r p r e t e d i n s t e a d as a s e l e c t i o n a l p rocess which reduced the d i f f e r e n t i a l in c o m p e t i t i v e a b i l i t i e s of the i n f e r i o r and s u p e r i o r components of i n t e r a c t i o n s i n younger communities. As t h i s study i n c l u d e d s e v e r a l dominant s p e c i e s from the study s i t e , i t p r e s c r i b e s the r o l e of b i o l o g i c a l accommodation in a community-wide context and lends f u r t h e r support to the proposed model f o r pasture community e v o l u t i o n ( F i g . 9, Chapter 3). 1 32 CHAPTER 5 CHANGES IN THE NATURE OF COMPETITIVE RELATIONS AMONGST THREE SPECIES IN DIFFERENT AGED PASTURES SUBSTITUTIVE REPLACEMENT SERIES MODEL 133 INTRODUCTION In a d i a l l e l design f o r studying c o m p e t i t i o n (Chapter 4), the performance i n a 1:1 mixture can r e v e a l a p a r t i c u l a r type of i n t e r a c t i o n between a p a i r of genotypes. The a n a l y s i s of that i n t e r a c t i o n can however be c a r r i e d f u r t h e r using data on mixtures having a range of p r o p o r t i o n s but m a i n t a i n i n g d e n s i t y constant ( i . e . a 'replacement s e r i e s ' ) . Based on these c r i t e r i a , de Wit (1960, 1961) i n t r o d u c e d experimental models which have been widely used fo r i d e n t i f y i n g and q u a n t i f y i n g the p r o p e r t i e s of c o m p e t i t i v e r e l a t i o n s h i p s between p l a n t s (Trenbath 1978). I t has been used fo r a n a l y z i n g how c o m p e t i t i v e i n t e r a c t i o n s are i n f l u e n c e d by environment; e.g. the e f f e c t of the presence of symbiotic Rhi zobi urn on the c o m p e t i t i v e r e l a t i o n s h i p between a grass and legume (de Wit et a l . 1966) and the e f f e c t of s o i l depth on d i f f e r e n t i a l root placement i n s p e c i e s mixtures of the genus Avena (Trenbath & Harper 1973). I t has a l s o been u s e f u l f o r i d e n t i f y i n g l i m i t i n g resources i n c o m p e t i t i v e i n t e r a c t i o n s ( H a l l 1974b). Information oh the frequency^dependence or independence of •: c o m p e t i t i v e i n t e r a c t i o n s permits i n t e r p r e t a t i o n of r e l a t i v e c o m p e t i t i v e a b i l i t i e s and degree of niche o v e r l a p between genotypes or s p e c i e s . The c o m p e t i t i v e advantage of oats sown i n a replacement s e r i e s with peas was found to be independent of the p r o p o r t i o n s i n mixture (de Wit 1960). In c o n t r a s t , mixtures of Anthoxanthum odoratum and Phleum pratense showed frequency- dependence with a c l o s e balance i n c o m p e t i t i v e a b i l i t y (van den Bergh & de Wit 1960). Frequency-dependent behaviour has been shown between T r i f o l i u m repens and Lolium perenne which i s 1 34 i n t e r p r e t e d as some degree of niche d i f f e r e n t i a t i o n i n these two s p e c i e s (Ennik 1960, de Wit 1961). The De Wit model p r o v i d e s a s e n s i t i v e method f o r a n a l y z i n g changes i n the c o m p e t i t i v e r e l a t i o n s of s p e c i e s i n response to s e l e c t i o n . Previous s t u d i e s have shown that p o p u l a t i o n s having a h i s t o r y of i n t e r a c t i o n g e n e r a l l y form more s t a b l e a s s o c i a t i o n s demonstrated by frequency-dependent behaviour in replacement s e r i e s a n a l y s i s than do p o p u l a t i o n s with no h i s t o r y of i n t e r a c t i o n (Remison & Snaydon - i n Snaydon 1978, Joy & L a i t i n e n 1980, M a r t i n & Harding 1981). These s t u d i e s present only c i r c u m s t a n t i a l evidence that niche d i f f e r e n t i a t i o n i s an evolved product of past c o m p e t i t i o n . No p r e v i o u s s t u d i e s using replacement s e r i e s a n a l y s i s have c o n s i d e r e d how the degree of niche o v e r l a p changes in a p a r t i c u l a r p a i r of s p ecies i n a p a r t i c u l a r s i t e (community). T h i s i s i n v e s t i g a t e d i n the present study using p l a n t s c o l l e c t e d from the study pastures belonging to a common developmental p r o g r e s s i o n . Changes in c o m p e t i t i v e r e l a t i o n s with i n c r e a s i n g pasture age are i n v e s t i g a t e d f o r . t h r e e d i f f e r e n t , p a i r s of s p e c i e s - Holcus l a n a t u s - T r i f o l i u m repens , Lolium perenne - T r i f o l i u m repens and Holcus lanatus - Lolium perenne. The s t u d i e s of e v o l u t i o n a r y changes in c o m p e t i t i v e r e l a t i o n s mentioned p r e v i o u s l y have used g e n e t i c a l l y n o n - i d e n t i c a l i n d i v i d u a l s (products of sexual reproduction) i n the replacement s e r i e s . In c o n t r a s t , the present study uses cloned ramets of 2 s i n g l e genets c o l l e c t e d as n a t u r a l neighbours growing in immediate p r o x i m i t y to one another i n the f i e l d . T h i s approach was taken i n an e f f o r t to d e t e c t the presence of l o c a l i z e d b i o t i c 135 s p e c i a l i z a t i o n of e s t a b l i s h e d genotypes and to i n v e s t i g a t e the developmental q u a l i t i e s of that s p e c i a l i z a t i o n as the community ages. In order to set the stage f o r the experimental design and f a c i l i t a t e i n t e r p r e t a t i o n of the r e s u l t s based on accepted theory, a b r i e f review of r e l e v a n t t h e o r e t i c a l c o n s i d e r a t i o n s w i l l f o l l o w . THEORY If mixtures of two genotypes or s p e c i e s are p l a n t e d at a v a r i e t y of p r o p o r t i o n s , the change i n the r a t i o of those p r o p o r t i o n s a f t e r an elapsed time can be p l o t t e d a g a i n s t the r a t i o of p r o p o r t i o n s at the beginning. The p l o t so produced - a ' r a t i o diagram'- pro v i d e s a s e n s i t i v e f i g u r a t i v e d i s p l a y f o r d e t e c t i n g the type of co m p e t i t i v e r e l a t i o n s h i p between two s p e c i e s . F i v e b a s i c types of i n t e r a c t i o n can be i n t e r p r e t e d ( F i g . 12) (de Wit 1961, Harper 1977b): a) ( F i g . 12a). The p r o p o r t i o n of the two s p e c i e s remains unchanged a f t e r a p e r i o d of growth together. Since the f i n a l p r o p o r t i o n r a t i o e x a c t l y equals the i n i t i a l p r o p o r t i o n r a t i o f o r a l l mixtures, i t i n d i c a t e s : ( a s s u m i n g d e n s i t i e s are high enough) that f o r both s p e c i e s , an i n t e r s p e c i f i c i n d i v i d u a l p r o v i d e s p r e c i s e l y the same i n t e n s i t y of competition as does an i n t r a s p e c i f i c i n d i v i d u a l , and that the two s p e c i e s make the same demands on environmental resources ( i . e . t h e i r niches broadly o v e r l a p ) . In t h i s case n e i t h e r s p e c i e s has any 'advantage'. In a l l c o n s i d e r a t i o n s below, an 'advantage' w i l l r e f e r to a c a p a c i t y i n a s p e c i e s to i n c r e a s e i t s frequency i n the f i n a l p r o p o r t i o n r a t i o r e l a t i v e to the i n i t i a l p r o p o r t i o n r a t i o . 136 FIGURE 12. R a t i o diagrams i l l u s t r a t i n g 5 p o s s i b l e outcomes from a replacement s e r i e s experiment r e f l e c t i n g 5 d i f f e r e n t types of c o m p e t i t i v e r e l a t i o n s h i p between s p e c i e s i and j . The dashed l i n e r e p r e s e n t s the t h e o r e t i c a l case where a l l i n i t i a l r a t i o s equal a l l f i n a l r a t i o s ( i . e . the l i n e of 'no advantage'). See te x t f o r d e s c r i p t i o n . 137 I n i t i a l l o g i / j 138 b) ( F i g . 12b). Here, s p e c i e s i gains an advantage in mixtures at a l l r e l a t i v e f r e q u e n c i e s . The most inte n s e c o m p e t i t i o n f o r j i s i n t e r s p e c i f i c , w h i l s t f o r i , i n t r a s p e c i f i c c o m p e t i t i o n i s more i n t e n s e . I f the slope of the l i n e i s 1.0 as shown, the advantage f o r i i s independent of frequency; i . e . an i n c r e a s e in the i n i t i a l p r o p o r t i o n r a t i o r e s u l t s i n a corresponding i n c r e a s e , by the same amount, in the f i n a l p r o p o r t i o n r a t i o , once again i n d i c a t i n g as i n a) that i and j o v e r l a p i n t h e i r use of a v a i l a b l e r e s o u r c e s . Hence, the advantage possessed by i i s r e f l e c t e d by a r e l a t i v e l y g r e a t e r c a p a c i t y to reduce the a v a i l a b i l i t y of resources to j from a supply on which they both make demands, and that t h i s c a p a c i t y i s the same r e g a r d l e s s of r e l a t i v e frequency i n mixture. A c c o r d i n g l y , the p e r p e n d i c u l a r d i s t a n c e of the a c t u a l r a t i o l i n e from the t h e o r e t i c a l l i n e of 'no advantage' i n d i c a t e s the magnitude of the s u p e r i o r i t y of i ' s r e l a t i v e c o m p e t i t i v e a b i l i t y . c) ( F i g . 12c). Here, the same p r i n c i p l e s as i n b) apply except that j gains an advantage in mixtures at a l l r e l a t i v e f r e q u e n c i e s . d) ( F i g . 12d). T h i s r e s u l t i n d i c a t e s a frequency-dependent c o m p e t i t i v e r e l a t i o n s h i p , i . e . which s p e c i e s has an advantage i s dependent on the i n i t i a l p r o p o r t i o n r a t i o . G e n e r a l l y , the m i n o r i t y component in a mixture i s always at an advantage - consequently the r e g r e s s i o n slope i s l e s s than 1.0. Here, in c o n t r a s t to b), each component experiences the most intense competition from an i n d i v i d u a l of i t s own s p e c i e s . Hence, the 'advantage' f o r the m i n o r i t y component of a mixture i s i n c u r r e d by i t s experience of fewer i n t r a s p e c i f i c i n d i v i d u a l s and hence 1 39 l e s s s t r i n g e n t c o m p e t i t i o n . With p l a n t s , t h i s t r a d i t i o n a l l y i m p l i e s that resource use by the two s p e c i e s does not completely o v e r l a p ; i and j have d i f f e r e n t n i c h e s . An e q u i l i b r i u m r a t i o occurs where the r e g r e s s i o n i n t e r s e c t s the t h e o r e t i c a l l i n e of 'no advantage'. At t h i s i n i t i a l p r o p o r t i o n r a t i o , an i d e n t i c a l f i n a l p r o p o r t i o n r a t i o r e s u l t s because, f o r both s p e c i e s , the l e v e l of i n t r a s p e c i f i c d e n s i t y imposes hardships of resource l i m i t a t i o n that are equal i n magnitude to those hardships imposed by i n t e r s p e c i f i c c o m p e t i t i o n f o r those elements of niche which do o v e r l a p . e) ( F i g . I2e). T h i s r e s u l t a l s o i n d i c a t e s a frequency- dependent c o m p e t i t i v e r e l a t i o n s h i p but i n t h i s i n s t a n c e , the m a j o r i t y component of a mixture i s always at an advantage, r e g a r d l e s s of s p e c i e s . Hence, the r e g r e s s i o n slope i s g r e a t e r than 1.0. I n t r a s p e c i f i c c o m p e t i t i o n i s never stronger than i n t e r s p e c i f i c c o m petition i n e i t h e r s p e c i e s . T h i s r e s u l t p r e d i c t s that e i t h e r s p e c i e s c o u l d out-compete the other depending on which has the h i g h e s t r e l a t i v e frequency i n mixture and an unstable e q u i l i b r i u m p o i n t t h e r e f o r e e x i s t s . Such.a s i t u a t i o n would occur i f each s p e c i e s impaired the environment of the other more than i t impaired i t s own environment, as f o r example with mutual a l l e l o p a t h i c i n t e r f e r e n c e (Harper 1977b). Here, the e f f e c t i v e n e s s of i n t e r f e r e n c e by one component on another encompasses a conce r t e d e f f o r t amongst c o h o r t s , and an 'advantage' f o r one component i s e n t i r e l y a consequence of higher r e l a t i v e numbers in the i n i t i a l p r o p o r t i o n r a t i o . U n l i k e b), the c a p a c i t y to reduce the a v a i l a b i l i t y of resources to the other component s p e c i e s changes with r e l a t i v e frequency i n 140 mixture, and t h i s a p p l i e s c o n c u r r e n t l y to both components. If in such an i n t e r a c t i o n , c o m p e t i t i v e e x c l u s i o n was imminent (as t h i s model p r e d i c t s ) , i t would imply that the extent of niche o v e r l a p was enough to make impossible any escape from e x t i n c t i o n . METHODS Three d i f f e r e n t genet p a i r s , Holcus l a n a t u s - T r i f o l i u m repens, Lolium perenne - T_̂  repens , and H^ lanat u s - L. perenne , were c o l l e c t e d as immediately adjacent neighbours from each of the 3 d i f f e r e n t aged pasture communities (2, 21 and 40 y e a r s ) . Each p a i r was c o l l e c t e d and propagated as o u t l i n e d i n Chapter 2 to o b t a i n a p o p u l a t i o n of ramets f o r each genet type. P l a s t i c pots (1.75 l i t e r - 15 cm diameter, 15 cm deep) c o n t a i n i n g a standard p o t t i n g mix of 50% peat, 25% sand and 25% p e r l i t e were used. For each n a t u r a l genet p a i r type, a constant d e n s i t y of 13 ramets per pot were p l a n t e d i n the replacement s e r i e s and arrangement shown i n F i g u r e 13. The p l a n t i n g •arrangement ensured that each ramet of the m i n o r i t y component ': type was surrounded, as much as p o s s i b l e , by ramets of the- m a j o r i t y component type. Each p r o p o r t i o n i n each replacement s e r i e s was r e p l i c a t e d three times and the pots were arranged randomly i n a p o l y e t h y l e n e t u n n e l . Rhizobium c u l t u r e was added when the ramets were p l a n t e d ( f i r s t week of June, 1980) and a l l pots r e c e i v e d 3 a p p l i c a t i o n s of l i q u i d organic f e r t i l i z e r (N/P/K : 10/5/5) ( a f t e r weeks 1, 5 and 9). The pots were watered whenever the s o i l s u r f a c e became dry, about 3 times weekly on average. A f t e r 4 weeks, an ' i n i t i a l ' count of the t o t a l number 141 FIGURE 13. P l a n t i n g arrangement used f o r 13 ramets i n the replacement s e r i e s : s p e c i e s _i 0 1 2 4 5 8 9 1 1 1 2 1 3 sp e c i e s j 1 3 1 2 1 1 9 8 5 4 2 1 0 142 5 / 8 143 of t i l l e r s ( f o r grasses) and measure of the t o t a l s t o l o n l e n g t h (f o r c l o v e r ) per pot were taken. T h i s f o l l o w s Ennik (1960). A f t e r a f u r t h e r 12 weeks, the experiment was terminated and a ' f i n a l ' t i l l e r count and s t o l o n measure per pot were recorded. RESULTS AND DISCUSSION For each replacement s e r i e s , i n i t i a l and f i n a l p r o p o r t i o n r a t i o s were c a l c u l a t e d . A f t e r l o g t r a n s f o r m a t i o n , the data were p l o t t e d as r a t i o diagrams ( F i g . 14) f o l l o w i n g m u l t i p l e l i n e a r r e g r e s s i o n a n a l y s i s using a MIDAS program (Fox & Guire 1976). R a t i o diagrams provide a convenient model f o r a n a l y z i n g changes in i n t e r s p e c i f i c c o m p e t i t i v e r e l a t i o n s . In accordance with e s t a b l i s h e d theory ( F i g . 12), the r a t i o diagrams produced from the present data may be i n t e r p r e t e d w i t h i n the context of a l t e r n a t i v e h y p o t h e t i c a l models f o r changing competitive r e l a t i o n s produced as a consequence of n a t u r a l s e l e c t i o n i n component s p e c i e s ( F i g . 15). These are c o n s i d e r e d below fo l l o w e d by a d i s c u s s i o n of p a r t i c u l a r s p e c i e s i n t e r a c t i o n s . Two c r i t i c a l v alues .are e x t r a c t e d from ratio."diagrams f o r i n t e r p r e t i n g c o m p e t i t i v e r e l a t i o n s : • 1) Changes i n the -slope • of the r e g r e s s i o n ; 2) p a r a l l e l changes in the p o s i t i o n of the r e g r e s s i o n . A de c r e a s i n g slope of the r e g r e s s i o n with a common po i n t of r o t a t i o n ( F i g . 15a), r e f l e c t s a process of niche d i f f e r e n t i a t i o n . The smal l e r the slo p e , the more d i f f e r e n t i a t e d are the two components with respect to niche requirements. With a slope of zero, the f i n a l r e l a t i v e frequency i s constant r e g a r d l e s s of i n i t i a l r e l a t i v e frequency, which i m p l i e s that the two components v i r t u a l l y a v o i d one another completely. The 1 44 FIGURE 14. R a t i o diagrams f o r the experimental r e s u l t s . Each p o i n t r e p r e s e n t s the value corresponding to the i n i t i a l and f i n a l r a t i o s i n v o l v i n g t o t a l s t o l o n l e n g t h and/or t o t a l t i l l e r number per s p e c i e s per pot. 145 146 -2.00 0 2.00 4.00 1 48 FIGURE 15. R a t i o diagrams showing t h e o r e t i c a l trends f o r e v o l u t i o n a r y changes i n c o m p e t i t i v e r e l a t i o n s between two s p e c i e s ( i . and j) dur i n g d i f f e r e n t stages (1, 2 and 3) of n a t u r a l s e l e c t i o n . See t e x t f o r d e s c r i p t i o n . 149 150 performance of each i s e n t i r e l y u n a f f e c t e d by the presence of the o t h e r . I t f o l l o w s that a reverse order of i n c r e a s i n g slope towards 1.0 would suggest s e l e c t i o n f o r niche convergence where, because of i n t e r a c t i o n , the f i n a l r e l a t i v e frequency very much depends on the i n i t i a l r e l a t i v e frequency. (This i s not to be confused with the f a c t that with a slope = 1.0, an advantage f o r a component in mixture i s independent of frequency.) An i n c r e a s i n g slope beyond 1.0 i n d i c a t e s a process of s e l e c t i o n f o r i n c r e a s i n g mutual i n t e r f e r e n c e e f f e c t s between the two components (not i l l u s t r a t e d ) . P a r a l l e l s h i f t s i n the p o s i t i o n of the r e g r e s s i o n in a frequency - independent model ( F i g . 15b), or a displacement of the e q u i l i b r i u m p o i n t from x to y to z i n a frequency dependent model ( F i g . 15c), r e f l e c t s changes in the r e l a t i v e c o m p e t i t i v e a b i l i t i e s of the two components. In both b) and c) the c o m p e t i t i v e a b i l i t y of j r e l a t i v e to i i s shown to be i n c r e a s i n g through stages 1, 2 and 3. C o n s i d e r a t i o n s above however d i c t a t e that i n b), i and j are using the same resource supply, whereas i n c) t h e i r niche requirements o v e r l a p l e s s . N e i t h e r b) nor c) i m p l i e s that s e l e c t i o n has changed the degree of niche o v e r l a p . E v o l u t i o n a r y changes i n both niche r e l a t i o n s h i p and r e l a t i v e c o m p e t i t i v e a b i l i t y are r e f l e c t e d by a concomitant change in the slope of the r e g r e s s i o n and s h i f t i n i t s p a r a l l e l p o s i t i o n ( F i g . 15d). For each s p e c i e s p a i r s t u d i e d , a n a l y s i s of v a r i a n c e r e v e a l e d the s i g n i f i c a n t d i f f e r e n c e s i n slopes and i n t e r c e p t s amongst genet p a i r s from d i f f e r e n t aged pastures ( F i g . 14). These comparisons are d i s p l a y e d i n F i g u r e 16 and a d i s c u s s i o n f o r each s p e c i e s p a i r f o l l o w s . 151 FIGURE 16. R a t i o diagrams f o r immediately adjacent genet p a i r s from the three d i f f e r e n t aged pastures (2, 21 and 40 y r s ) superimposed on the same graph. F - t e s t s f o r the homogeneity of v a r i a n c e s a f t e r l o g - t r a n s f o r m a t i o n were not r e j e c t e d . P r o b a b i l i t y l e v e l s are from an a n a l y s i s of v a r i a n c e f o r s i g n i f i c a n t d i f f e r e n c e s i n slopes and i n t e r c e p t s of the r e g r e s s i o n s (* s i g n i f i c a n t at P<0.05) f o r a l l p o s s i b l e p a i r w i s e comparisons of f i e l d age. A c o e f f i c i e n t of dete r m i n a t i o n i s given f o r each m u l t i p l e l i n e a r r e g r e s s i o n . Pasture age: 2 year; 21 year; 40 year. 152 / 1 1 1 1 r — — i 1 1 -2.00 0 2.00 4.00 Log (T r i f o l i um/Ho l cus ) ( I n i t i a l ) -2J 00 0 ' 2T0O ' 4.00 Log (T r i f o l i um/Lo l i um) ( I n i t i a l ) Log g (Holcus/Lol ium) ( I n i t i a l ) 153 a) Holcus lanat u s - T r i f o l i u m repens ( F i g . 16a). H. l a n a t u s and repens showed a trend of d e c r e a s i n g r e g r e s s i o n slope i n the r a t i o diagram as w e l l as a s h i f t i n the p o s i t i o n of the r e g r e s s i o n ( i . e . as i n F i g u r e I5d). An unstable behaviour (slope g r e a t e r than 1.0) i s i l l u s t r a t e d f o r the two year o l d genet p a i r and t h i s s h i f t s to a s t a b l e behaviour (slope l e s s than 1.0) i n the p a i r s from the two o l d e r p a s t u r e s . T h i s i s i n t e r p r e t e d as niche divergence. The 21 year and 40 year p a i r s do not s i g n i f i c a n t l y d i f f e r i n r e g r e s s i o n slope but do d i f f e r i n p o s t i o n ( i n t e r c e p t ) . The r e g r e s s i o n s h i f t s p r o p o r t i o n a l l y to favour T\ repens at a l l r e l a t i v e f r e q u e n c i e s . T h i s i n d i c a t e s a higher r e l a t i v e c o m p e t i t i v e a b i l i t y f o r T r i f o l i u m i n the 40 year p a i r than i n the 21 year p a i r but no d i f f e r e n c e i n amount of niche o v e r l a p between the two components. b) Lolium perenne - Tr i f o l i u m repens ( F i g . 16b). In a l l cases the c o m p e t i t i v e r e l a t i o n s h i p between these two s p e c i e s i s frequency-dependent (with slope < 1.0) but the slope of the r e g r e s s i o n i s higher, i n p a i r s from the two older., p a s t u r e s . T h i s i n d i c a t e s that niche o v e r l a p i s g r e a t e r between genet p a i r s from the two o l d e r p a s t u r e s than from the youngest pasture suggesting a s e l e c t i o n process of niche convergence. The p o s i t i o n of the r e g r e s s i o n a l s o changes, again demonstrating an example of model d i n F i g u r e 15; i . e . the 21 year and 40 year p a i r s do not s i g n i f i c a n t l y d i f f e r i n r e g r e s s i o n slope but do d i f f e r i n p o s i t i o n . As with lanatus and T\ repens, the r e g r e s s i o n f o r perenne and T^ repens s h i f t s p r o p o r t i o n a l l y to favour T\ repens at a l l r e l a t i v e f r e q u e n c i e s . T\ repens 1 54 e v i d e n t l y has a higher r e l a t i v e c o m p e t i t i v e a b i l i t y a g a i n s t L. perenne f o r common niche requirements i n the 40 year pasture than i n the 21 year p a s t u r e . c) Holcus l a n a t u s - Lolium perenne ( F i g . 16c). These two species demonstrate l a r g e l y a frequency- independent c o m p e t i t i v e r e l a t i o n s h i p i n a l l p a i r s . T h i s i n d i c a t e s that i n a l l 3 p a s t u r e s , H^ l a n a t u s and L_;_ perenne have widely o v e r l a p p i n g niche requirements and t h i s degree of o v e r l a p does not change as there i s no s i g n i f i c a n t d i f f e r e n c e i n the three r e g r e s s i o n s l o p e s . H_;_ l a n a t u s has an advantage i n mixtures at a l l r e l a t i v e f r e q u e n c i e s but t h i s advantage d i m i n i s h e s with i n c r e a s i n g pasture age; the 40 year r e g r e s s i o n approaches the t h e o r e t i c a l l i n e of 'no advantage' and i s s i g n i f i c a n t l y d i f f e r e n t i n i n t e r c e p t from the 2 year r e g r e s s i o n . T h i s i n d i c a t e s that the extent to which H^ la n a t u s i s the s u p e r i o r competitor of the two i s l e s s i n p l a n t s from o l d e r pastures (model b i n F i g u r e 15). The t r a d i t i o n a l e x p e c t a t i o n of c o m p e t i t i v e i n t e r a c t i o n between•. spec i e s i s divergence.'': Antonovics "'( 1 978 ) reviews the f i n d i n g s of experimental' s t u d i e s on changes in c o m p e t i t i v e performance as a r e s u l t of c o m p e t i t i o n , most of which i n v o l v e D r o s o p h i l a . S e v e r a l s t u d i e s have demonstrated changes, as t h e o r e t i c a l l y expected, i n the d i r e c t i o n of i n c r e a s e d d i f f e r e n t i a l resource u t i l i z a t i o n by one or both components and g r e a t e r r e p r o d u c t i v e output of the mixture (Moore 1952, Seaton & Antonovics 1967, van Deldon 1970, Barker 1973, Chen 1973 ( c i t e d i n Antonovics 1978)) . Other work has shown that c o m p e t i t i v e performance remains unchanged (Sokal et a l . 1970, Hedrick 1 55 1973, Sulzbach 1980), or in some cases even d e c l i n e s (Futuyma 1970, Ford 1972). V a r i a b l e r e s u l t s may be due to u n p r e d i c t a b i l i t y a s s o c i a t e d with experimental designs, or due to lack of g e n e t i c v a r i a n c e or c o n s t r a i n t s on s e l e c t i o n caused by such f a c t o r s as negative c o r r e l a t i o n among f i t n e s s components or i n b r e e d i n g depression (Antonovics 1978). V a r i a b l e r e s u l t s however may a l s o r e s u l t i f s e l e c t i o n operates i n a v a r i e t y of d i f f e r e n t ways and the query which a r i s e s when r e s u l t s c o n t r a d i c t ' t r a d i t i o n a l e x p e c t a t i o n ' may simply be a consequence of an erroneous e x p e c t a t i o n . S t u d i e s f o r p l a n t s comparable to the above demonstrations of the p r o g r e s s i o n of change in c o m p e t i t i v e r e l a t i o n s are unknown p r i o r to the present i n v e s t i g a t i o n . U n l i k e previous s t u d i e s , the present work addresses the q u e s t i o n : What i s the c o m p e t i t i v e r e l a t i o n s h i p between e s t a b l i s h e d genet p a i r s that are a c t u a l l y i n t e r a c t ing i n nature and how does t h i s r e l a t i o n s h i p d i f f e r ( f o r the same two s p e c i e s ) i n communities whose p o p u l a t i o n s have had d i f f e r e n t lengths of time to respond to the c o m p e t i t i v e environment? Furthermore, in the present study, i n t e r p r e t a t i o n i n v o l v e s s e l e c t i o n which has o c c u r r e d in nature, not under a r t i f i c i a l l a b o r a t o r y c o n d i t i o n s . I n d i v i d u a l s which i n t e r a c t i n the experiments rep o r t e d here are of i d e n t i c a l genotype to those i n d i v i d u a l s which were a c t u a l l y i n t e r a c t i n g i n the f i e l d . R e s u l t s which do not meet t r a d i t i o n a l e x p e c t a t i o n were found and i n t e r p r e t a t i o n s are o f f e r e d which r e f l e c t d i f f e r e n t ways i n which n a t u r a l s e l e c t i o n may operate ( i . e . F i g u r e 15). lanatus and T\ repens show evidence s u p p o r t i n g an h y p o t h e s i s of niche divergence as t r a d i t i o n a l l y expected, but 1 56 L. perenne and repens show evidence f o r niche convergence and a l l 3 s p e c i e s p a i r s show evidence suggesting that s e l e c t i o n has changed the r e l a t i v e c o m p e t i t i v e a b i l i t i e s of the s p e c i e s , a n o n - i d e n t i c a l event to changes i n the extent of niche o v e r l a p . R e s u l t s f o r the two grasses ( F i g . 16c), }U_ l a n a t u s and L. perenne , suggest that s e l e c t i o n has r e s u l t e d s o l e l y i n a more balanced r e l a t i v e c o m p e t i t i v e power between the two s p e c i e s f o r a resource supply on which they both make demands. The unusual r e s u l t suggesting niche convergence between L. perenne and T\_ repens suggests the p o s s i b i l i t y of some form of beneficence between these two s p e c i e s . T h i s may be r e l a t e d to a n i t r o g e n - r i c h environment made a v a i l a b l e to the grass through c l o s e a s s o c i a t i o n with the c l o v e r (Wilson 1942, V a l l i s 1978). Evidence f o r a strong t r e n d of i n c r e a s i n g s t a b l e a s s o c i a t i o n between these two s p e c i e s with i n c r e a s i n g pasture age was presented in Chapter 3. In r e s p e c t to animal c o m p e t i t i o n , MacArthur & Levins (1967) suggest that convergence may r e s u l t between two s p e c i e s i f t h e i r a c t i v i t y together i s more e f f e c t i v e i n suppressing a t h i r d more a g g r e s s i v e competitor common . to both. Convergence i s a l s o a s s o c i a t e d , with the kinds of s e l e c t i o n f o r c e s a s s o c i a t e d with mimicry (Sheppard 1975). Agren & Fagerstrom (1980) propose a model of niche convergence f o r two p l a n t s p e c i e s which i n i t i a l l y d i f f e r i n t h e i r f l o w e r i n g times. They argue that a competitor A, which i s s u p e r i o r to competitor B i n competition f o r p o l l i n a t o r s , but i n f e r i o r i n s e e d l i n g competition , may be able to i n c r e a s e i t s r e l a t i v e f i t n e s s by e v o l v i n g i n c r e a s e d niche o v e r l a p with respect to f l o w e r i n g time, and thereby reducing seed p r o d u c t i o n of competitor B. Such 157 c o n s i d e r a t i o n s draw a t t e n t i o n away from t r a d i t i o n a l pre- occupation with 'divergence' as a consequence of competitive i n t e r a c t i o n , and focus i n s t e a d on the r o l e of r e l a t i v e c o m p e t i t i v e power and concomitant b e n e f i c i a l i n t e r a c t i o n s i n the e v o l u t i o n a r y adjustment of s p e c i e s to t h e i r environment of neighbours. In t h i s v e i n , a f i n e r - s c a l e i n v e s t i g a t i o n of the i n t e r a c t i o n between perenne and T\_ repens w i l l be presented in the next chapter. 158 CHAPTER 6 BIOTIC SPECIALIZATION AT THE GENOTYPE LEVEL: RECIPROCAL PHYTOMETER TRANSPLANTS AMONGST FOUR NATURAL NEIGHBOURING GENET' PAIRS OF LOLIUM PERENNE AND TRIFOLIUM REPENS 159 INTRODUCTION Pop u l a t i o n ecology i s to a l a r g e extent the study of n a t u r a l s e l e c t i o n . Genetic v a r i a t i o n d i c t a t e s that some i n d i v i d u a l s leave more descendents than o t h e r s . The p r o p e r t i e s of a l l l e v e l s of b i o l o g i c a l o r g a n i z a t i o n are consequently a f f e c t e d and the study of ecology asks how and why of the r e l a t e d processes and r e s u l t a n t p a t t e r n s . Since 1967, Harper (1967, 1978) has c a l l e d a t t e n t i o n to the need for more i n t e r a c t i o n between p o p u l a t i o n g e n e t i c s and p o p u l a t i o n ecology and has s t r e s s e d that the convergence of the two p r o v i d e s one of the most e x c i t i n g f i e l d s of development in modern b i o l o g y . Such t h i n k i n g has had recent consequences at the l e v e l of community ecology. E c o l o g i s t s have become d i s s a t i s f i e d with the taxonomists' ' t y p o l o g i c a l ' c a t e g o r i z a t i o n of the s p e c i e s as these c a t e g o r i e s were formulated with d i f f e r e n t needs and p h i l o s o p h i e s i n mind than those of e c o l o g i s t s (Antonovics 1976a). I n t e r e s t has been a i r e d i n recent l i t e r a t u r e to regard the genotype as a r e l e v a n t u n i t of community d i v e r s i t y and to view the community as a conglomerate of e v o l v i n g components (Antonovics 1976a, Harper 1977b, 1982). These components may be regarded as l o c a l neighbourhoods w i t h i n the community in which, not the p a r t i c u l a r s p e c i e s , but the p a r t i c u l a r genotypes of d i f f e r e n t neighbours d i c t a t e the events which att e n d b i o t i c i n t e r a c t i o n and set the i t i n e r a r y of community e v o l u t i o n ( F i g . 9, Chapter 3). Most demonstrations of a d a p t i v e genetic v a r i a t i o n i n p l a n t s are not community-based s t u d i e s ; they concern a s i n g l e component p o p u l a t i o n from a d i v e r s e s p e c i e s assemblage. I n v e s t i g a t i o n s 160 have shown that i f d i s r u p t i v e s e l e c t i o n i s strong enough, d i f f e r e n t adaptive 'types' can a r i s e w i t h i n a s i n g l e gene pool even over very short d i s t a n c e s and i n s p i t e of the d i l u t i n g e f f e c t from gene flow ( J a i n & Bradshaw 1966, E h r l i c h & Raven 1969, Bradshaw 1972). In most s t u d i e s , the environments d i s c u s s e d concern s t r i n g e n t a b i o t i c s e l e c t i v e f o r c e s , such as wide ranges in the degree of s a l i n i t y i n maritime h a b i t a t s (Aston & Bradshaw 1966), or l e v e l s of heavy metals (Antonovics et a l . 1971) or f e r t i l i z e r s (Snaydon & Davies 1972) i n s o i l s . Very l i t t l e a t t e n t i o n has been given to the r o l e of such adap t i v e g e n e t i c v a r i a t i o n i n c o n s i d e r a t i o n s of s t r u c t u r e and e v o l u t i o n at the community l e v e l . To the p l a n t e c o l o g i s t a community i s e s s e n t i a l l y a group of c o - o c c u r r i n g s p e c i e s on the same t r o p h i c l e v e l amongst which some degree of i n t e r a c t i o n i s presumed. I t i s reasonable t h e r e f o r e to expect that the s t r u c t u r e and e v o l u t i o n of p l a n t communities w i l l to a l a r g e extent be determined by the q u a n t i t y and q u a l i t y of g e n e t i c v a r i a t i o n r e l a t e d to the p r o p e r t i e s of 'con-trophic', b i o t i c . i n t e r a c t i o n s . . In s p i t e . of t h i s , s u r p r i s i n g l y l i t t l e work has been done to study such v a r i a t i o n i n p l a n t communities. Any community-level i n t e r p r e t a t i o n of a d e s c r i p t i v e v e g e t a t i o n study based on the b i o l o g y of the c o n s t i t u e n t s p e c i e s r e q u i r e s an examination of the r e c i p r o c a l manner i n which neighbouring i n d i v i d u a l s respond to one another. Most s t u d i e s which have shown m i c r o - e v o l u t i o n i n p l a n t p o p u l a t i o n s i n response to b i o t i c i n t e r a c t i o n s however, have again been concerned with d i f f e r e n t i a l behaviour (responses) w i t h i n a s i n g l e s p e c i e s . For example, (Watson 1969) found that 161 p o p u l a t i o n s of P o t e n t i l l a e r e c t a growing i n neighbouring areas of M o l i n i a - and A g r o s t i s - dominated g r a s s l a n d , were d i f f e r e n t in respect to s i z e of c h a r a c t e r s when compared as both t r a n s p l a n t m a t e r i a l and as seed progeny growing i n an experimental garden. L i n h a r t (1974) d i s c o v e r e d d i f f e r e n t i a t i o n between p e r i p h e r a l i n d i v i d u a l s and i n d i v i d u a l s i n the c e n t r a l p o r t i o n of a s i n g l e p o p u l a t i o n of V e r o n i c a p e r e g r i n a over a d i s t a n c e of only two to f i v e meters. The d i f f e r e n t i a t i o n appeared to be r e l a t e d to intense i n t r a - s p e c i f i c c o m p e t i t i o n i n the c e n t r a l region versus more intense i n t e r - s p e c i f i c c o m p etition near the p e r i p h e r y . Watson (1974, c i t e d i n Antonovics 1978) found that with respect to s e v e r a l growth c h a r a c t e r s , Plantago l a n c e o l a t a c o l l e c t e d from d i f f e r e n t micro- h a b i t a t s were g e n e t i c a l l y d i f f e r e n t i a t e d with respect to the v e g e t a t i o n height from which they came. Turkington & Harper (1979c) found evidence f o r m i c r o - e v o l u t i o n i n a p o p u l a t i o n of T r i f o l i u m repens in response to d i f f e r e n t s e l e c t i o n p r e s s u r e s exerted by d i f f e r e n t s p e c i e s of grass i n l o c a l i z e d regions of a permanent p a s t u r e . B i o t i c ecotypes w i t h i n p o p u l a t i o n s are evidence of t r u l y 'Darwinian' f o r c e s of n a t u r a l s e l e c t i o n i n nature. S t u d i e s d i s c u s s e d above have demonstrated f i n e - s c a l e g e n e t i c a l l y - b a s e d b i o t i c s p e c i a l i z a t i o n at the s p e c i e s l e v e l w i t h i n s i n g l e p o p u l a t i o n s . A community l e v e l context f o r m i c r o - e v o l u t i o n that focuses on the genotype as a r e l e v a n t u n i t of d i v e r s i t y r a i s e s f u r t h e r , more e l a b o r a t e q u e s t i o n s : Does there e x i s t l o c a l concomitant ( i . e . r e c i p r o c a l ) b i o t i c s p e c i a l i z a t i o n i n each of two i n t e r a c t i n g s p e c i e s i n a community? Does s p e c i a l i z a t i o n 162 occur at the genotype l e v e l ? - i . e . does m i c r o - e v o l u t i o n occur d i f f e r e n t i a l l y w i t h i n a s i n g l e p o p u l a t i o n i n response to d i f f e r e n t neighbouring genotypes of a s i n g l e s p e c i e s , and i s there r e c i p r o c a l s p e c i a l i z a t i o n i n the neighbouring genotypes? A l l a r d & Adams (1969) showed that l i n e s of b a r l e y that had p e r s i s t e d together i n mixture over many genera t i o n s on average y i e l d e d higher i n mixture than i n pure stands. In c o n t r a s t , l i n e s that had no h i s t o r y of i n t e r a c t i o n g e n e r a l l y showed no d i f f e r e n c e i n y i e l d s i n mixture compared to pure stands. Joy and L a i t i n e n (1980) d i s c o v e r e d a higher y i e l d advantage i n mixture (over pure components) in s t r a i n s of Phleum pratense and T r i f o l i u m pratense that had been c u l t i v a t e d i n mixture f o r s e v e r a l generations than i n a mixture of two other randomly chosen c u l t i v a r s of the same s p e c i e s . In what way(s) does s e l e c t i o n a d j u s t the behaviour of p l a n t s in such b i o t i c s p e c i a l i z a t i o n at the genotype l e v e l i n g r a s s - c l o v e r combinations? Does i t i n v o l v e e v o l u t i o n a r y changes in both i n t e r a c t i n g p o p u l a t i o n s ? What r o l e might a p o s s i b l e b e n e f i c i a l i n t e r a c t i o n . of commensalism (from the n i t r o g e n - r i c h environment pr o v i d e d by the legume), e x i s t i n g c o n c u r r e n t l y with the negative i n t e r a c t i o n of c o m p e t i t i o n , have i n e s t a b l i s h i n g the e v o l u t i o n a r y response to neighbour i n t e r a c t i o n s ? These q u e s t i o n s underscore a view of the p l a n t community that i s centered on the c o e v o l u t i o n of p l a n t competitors r e c i p r o c a l e v o l u t i o n a r y responses to competition i n v o l v i n g mutual s h i f t s i n g e n e t i c c o n s t i t u t i o n s of i n t e r a c t i n g s p e c i e s a phenomenon that has r e c e i v e d l i t t l e a t t e n t i o n i n e v o l u t i o n a r y t h i n k i n g and v i r t u a l l y ignored i n pasture ecology (Antonovics 1 63 1976b, Snaydon 1978). Information about the p r o p e r t i e s of b i o t i c i n t e r a c t i o n s and ensuing m i c r o - e v o l u t i o n a r y p a t t e r n s i s incomplete without an i n v e s t i g a t i o n of r e c i p r o c a l responses between neighbours. R e c i p r o c a l responses of n a t u r a l neighbours d e f i n e s the 'combining a b i l i t y ' (Chapter 4) of i n d i v i d u a l s which a c t u a l l y i n t e r a c t i n the community. Combining a b i l i t y r e f l e c t s the extent to which each component i s compatible i n the presence of the other; i t r e f e r s to the c a p a c i t y of two s p e c i e s or genotypes to continue i n t e r a c t i o n with one another, or to c o e x i s t (Chapter 3). T r a d i t i o n a l theory t a c i t l y assumes that c o e x i s t e n c e in systems of c o m p e t i t i o n r e q u i r e s some measure of niche d i f f e r e n t i a t i o n . The p r e f i x e d term, ' e c o l o g i c a l ' combining a b i l i t y (Harper 1964) a l l u d e s to t h i s mechanistic i n t e r p r e t a t i o n ; s p e c i e s with e c o l o g i c a l combining a b i l i t y , by d e f i n i t i o n , c o e x i s t because t h e i r niches do not broadly o v e r l a p (Harper 1977b). T h i s does not n e c e s s a r i l y imply that a l l i n t e r a c t i o n i s precluded, but r a t h e r that i n t e r a c t i o n does not take p l a c e i n respect to any elements or r e q u i s i t e s of niche i n e i t h e r s p e c i e s , f o r which an ensuing c o n t e s t would lead to the c o m p e t i t i v e e x c l u s i o n of one by the other. In the work presented here, l o c a l p o p u l a t i o n d i f f e r e n t i a t i o n at the genotype l e v e l i s s t u d i e d i n respect to r e c i p r o c a l responses between n a t u r a l genotype neighbours from i n t e r a c t i n g p o p u l a t i o n s of Lolium perenne and T r i f o l i u m repens . Previous s t u d i e s have shown that d i f f e r e n t genotypes of a grass s p e c i e s d i f f e r i n t h e i r response to T r i f o l i u m repens (Myers & Garber 1942, Ahlgren et a l . 1945). D i f f e r e n t c l o n e s of c l o v e r have a l s o been shown to vary i n response to a p a r t i c u l a r grass 1 64 (Turkington et a l . 1979, Turkington & Harper 1979c). T. repens i n the present study s i t e showed a p r o g r e s s i o n of i n c r e a s i n g s t a b i l i t y of a s s o c i a t i o n with Lolium perenne in o l d e r pastures ( F i g . 8, Chapter 3). The replacement s e r i e s experiment showed evidence f o r niche convergence between these two s p e c i e s and i n c r e a s e d r e l a t i v e c o m p e t i t i v e a b i l i t y of T. repens i n o l d e r pastures (Chapter 5). The extent of s p e c i a l i z a t i o n and the nature of the combining a b i l i t y of these two s p e c i e s i s f u r t h e r i n v e s t i g a t e d here. The experiment was designed to t e s t whether samples of Lolium and T r i f o l i u m c l o n e s taken as neighbouring p a i r s from d i f f e r e n t l o c a l i t i e s i n the o l d e s t (1939) pasture d i f f e r e d i n t h e i r a b i l i t y to grow i n the presence of each other. METHODS Experimental Design Four d i f f e r e n t genet p a i r s of Lolium perenne and T r i f o l i u m repens (designated L1-T1, L2-T2, L3-T3, L4.-T4) were c o l l e c t e d as p h y s i c a l neighbours from four widely separate l o c a t i o n s i n the 1939 pasture where there was abundant o v e r l a p i n t h e i r percentage cover f r e q u e n c i e s . The p a i r s were c o l l e c t e d as two whole p l a n t s ( i n c l u d i n g root m a t e r i a l ) and each genet 'type' was propagated s e p a r a t e l y under glasshouse c o n d i t i o n s f o r 4 months by p e r i o d i c s e p a r a t i o n of t i l l e r s or c u t t i n g of s t o l o n p i e c e s followed by r e p l a n t i n g as ramets of the o r i g i n a l c l o n e . P l a s t i c pots (26.5 cm top diameter; 30.5 cm deep) were used c o n t a i n i n g a standard p o t t i n g mix of 50% peat, 25% sand and 25% p e r l i t e with 165 added n u t r i e n t s s p e c i f i e d i n Table 15. N u t r i e n t s were added at h a l f the recommended r a t e to ensure that they were not a v a i l a b l e in abundance. Each pot ( F i g . 17) had a 2.5 cm border i n which seeds of D a c t y l i s glomerata were sown at a high d e n s i t y to serve as an outer b u f f e r zone and to ' c o n t a i n ' the study p l a n t s by p r e v e n t i n g c l o v e r s t o l o n s from growing out over the edge of the pot and keeping leaves from adjacent pots from i n t e r a c t i n g . The a p p r o p r i a t e height to achieve t h i s c o u l d be monitored by v a r y i n g the height at which the D a c t y l i s border was kept c l i p p e d . The D a c t y l i s border zone was separated below ground from the c e n t r a l zone by a p l a s t i c ' s l e e v e ' . TABLE 15. N u t r i e n t composition of p o t t i n g mix used f o r the c o m p e t i t i o n experiment between d i f f e r e n t genets of Lolium perenne and T r i f o l i u m repens . (Values are per 35 l i t e r s of p o t t i n g mix). 83 g osmocote (18-6-12) (9 month slow r e l e a s e ) 109 g dolomite lime 43 g gypsum 4.5 g f r e t t e d t r a c e elements (boron, copper, i r o n , manganese^ molybdenum, z inc) Three weeks a f t e r sowing the D a c t y l i s border, 8 ramets of each grass 'type' were p l a n t e d with 8 ramets of each c l o v e r 'type' in a l l p o s s i b l e p a i r e d combinations. The p l a n t i n g arrangement had 4 c e n t r a l phytometers (2 of each s p e c i e s ) surrounded by the remaining ramets which served as an inner b u f f e r zone ( F i g . 17). The 4 c e n t r a l phytometers t h e r e f o r e 1 66 FIGURE 17. P l a n t i n g arrangement f o r pots c o n t a i n i n g d i f f e r e n t genet combinations of Lolium perenne (L) and T r i f o l i u m repens ( T ) . The outer 2.5 cm border was sown with D a c t y l i s glomerata. The cummulative y i e l d of the 4 denoted c e n t r a l phytometers was fo l l o w e d i n the experiment, each s p e c i e s component being recorded s e p a r a t e l y . 167 168 experienced the most r e p r e s e n t a t i v e b i o t i c environment - i . e . each c e n t r a l phytometer was surrounded by 4 phytometers of the other s p e c i e s . Each combination of c l o v e r 'type' and grass 'type' was r e p l i c a t e d three times and the pots were arranged randomly i n a temperature c o n t r o l l e d g l a s s house. The pots were watered whenever the s o i l s u r f a c e became dry, about 2 to 3 times weekly on average. Once every 3 to 4 weeks the p l a n t s ( i n c l u d i n g the D a c t y l i s border) were c l i p p e d to approximately 3 cm to simulate g r a z i n g . The c l i p p i n g s from the 4 c e n t r a l phytometers were c o l l e c t e d each time, d r i e d and weighed s e p a r a t e l y a c c o r d i n g to s p e c i e s . Twelve months a f t e r p l a n t i n g the ramets, the t o t a l above ground p a r t s of the 4 c e n t r a l phytometers were harvested, d r i e d and weighed and the weights of the two components were added to the r e s p e c t i v e cumulative t o t a l s . Data A n a l y s i s An index of 'combining a b i l i t y ' was c a l c u l a t e d f o r each L-T genet combination as f o l l o w s (see Chapter 4 ): CA = y/Y' where Y i s the observed value (per pot) f o r the lower y i e l d i n g component and Y' i s the observed value f o r the higher y i e l d i n g component. (In each combination, T\_ repens assumed the value of Y and perenne assumed the value of Y ' ). The CA index i s compared amongst genet combinations and t h e r e f o r e has s t r i c t l y r e l a t i v e v a l u e . I t assumes that a measure of the r e l a t i v e c o n t r i b u t i o n of the two components to the combined y i e l d r e f l e c t s t h e i r c a p a c i t y to p e r s i s t in i n t e r a c t i o n r e l a t i v e t o the same measure f o r other combinations. Data were t a b u l a t e d i n 169 4 rows and 4 columns as i n Table 16 f o r grass y i e l d s , c l o v e r y i e l d s , t o t a l y i e l d s and combining a b i l i t y indexes. Each of these v a r i a b l e s was subjected to a m u l t i p l e l i n e a r r e g r e s s i o n a n a l y s i s using the f o l l o w i n g l i n e a r model (Table 16): v.., = u'+r. + c • + 6 • • d- + e - - i where, y. ., i s a value f o r the observed v a r i a b l e ( i . e . t o t a l y i e l d , component y i e l d or CA index); i i s the row number; j i s the column number; k i s the r e p l i c a t e number; y i s the mean of the c e l l i n row 1, column 1 (assuming no d i a g o n a l e f f e c t ) ; r i i s a v a r i a b l e f o r 'row e f f e c t ' ( i . e . component of the o b s e r v a t i o n due to the e f f e c t of row i ) ; c.. i s a v a r i a b l e f o r 'column e f f e c t ' ( i . e . component of the o b s e r v a t i o n due to the e f f e c t of column j ) ; d i i s a v a r i a b l e f o r 'diagonal e f f e c t ' ( i . e . component of the o b s e r v a t i o n due to the e f f e c t of p r i n c i p a l d i a g o n a l p o s i t i o n i ) ; 6 has a value of 1 i f i=j and a value of 0 i f i ^ j ( i . e . i n c o r p o r a t e a d i a g o n a l e f f e c t i f the o b s e r v a t i o n i s i n a p r i n c i p l e d i a g o n a l c e l l ) ; e i j k i s a random normal v a r i a t e . The o v e r a l l t e s t had the n u l l h y p o t h e s i s : Ho: r2 = r3 = r4 = c2 = c3 = c4 = d1 = d2 = d3 .= d4 = 0 ( i . e . no row, column or d i a g o n a l e f f e c t s ) . A n a l y s i s was aimed p r i m a r i l y to d e t e c t whether there were any s i g n i f i c a n t d i a g o n a l e f f e c t s i n the data, a l l o w i n g a l s o f o r any p o s s i b l e row or column e f f e c t s . Where the a n a l y s i s r e v e a l e d no s i g n i f i c a n t row or column e f f e c t s , an o v e r a l l t e s t was generated with the n u l l h y p o t h e s i s : Ho: d1 = d2 = d3 = d4 = 0 ( i . e . assuming row and column e f f e c t s are z e r o ) . For 170 TABLE 16. Tabular model used i n the a n a l y s i s of v a r i a n c e (see te x t f o r d e s c r i p t i o n ) . CLOVER TYPE T l T2 T3 T4 LI y + d l y + c 2 y + C3 11 + C4 y + r + TY PE  L2 V + r2 C2 z + d2 v + r2 + C3 y + r 2 + c 4 CO CO y + r, + L3 y + r3 y + r3 + C2 o C3 + d3 y + r 3 + c 4 y + r 4 + C4 °4 L4 y + r4 y + r4 + C2 V + T A * C3 171 i n d i v i d u a l d i a g o n a l e f f e c t s the f o l l o w i n g n u l l hypotheses were t e s t e d : H o i : d1 = 0, Ho2: d2 = 0, Ho3: d3 = 0, Ho4: d4 = 0. Data were analyzed using a MIDAS program (Fox & Guire 1976). F - t e s t s f o r the homogeneity of v a r i a n c e s were not r e j e c t e d and a l l analyses were performed on untransformed data. RESULTS The r e s u l t s of dry weight p r o d u c t i o n f o r T r i f o l i u m and Lolium i n a l l g e n e t - p a i r combinations are shown in Tables 17 and 18 r e s p e c t i v e l y . The n u l l h ypothesis of no row, column or d i a g o n a l e f f e c t s was r e j e c t e d i n both cases. For each genet- p a i r type Lx-Tx ( F i g . 18), the y i e l d of T ( c l o v e r ) i n any combination i n v o l v i n g Lx or Tx was g e n e r a l l y highest in the n a t u r a l neighbouring combination; L3-T3 and L4-T4 showed s i g n i f i c a n t (P<0.05) d i a g o n a l e f f e c t s . The converse was true for Lolium ; y i e l d was lowest i n the n a t u r a l neighbouring genet combination and s i g n i f i c a n t d i a g o n a l e f f e c t s were found f o r L1- •T1 and L3-T3 .' ( F i g . 18 ) . • There was l i t t l e - neighbour s p e c i f i c p a t t e r n f o r 'total (L p l u s T) y i e l d s of the combinations (Table 19); the L1-T1 combination showed the only s i g n i f i c a n t d i a g o n a l ef f e c t . The f i n e s c a l e nature of b i o t i c s p e c i a l i z a t i o n between Lolium and T r i f o l i u m genotypes i s f u r t h e r evident from a comparison of combining a b i l i t y i n d i c e s ( F i g . 19 and Table 20). T h i s s i m ultaneously i n c o r p o r a t e s the n e i g h b o u r - s p e c i f i c i t y of L f o r T and T f o r L. For each genet p a i r type Lx-Tx, the combining a b i l i t y was higher than i n any other combination 172 TABLE 17. Y i e l d s (g) of phytometers of T r i f o l i u m repens (Tx), each c o l l e c t e d with, a n a t u r a l neighbouring genet of Lolium perenne (Lx) from four s i t e s i n the 1939 pasture and p l a n t e d i n a l l combinations of c l o v e r type and grass type; the performances of the c l o v e r types with with t h e i r n a t u r a l neighbouring grass types are shown on the p r i n c i p a l d i a g o n a l ; a l l values are means of 3 r e p l i c a t e s ; the p r o b a b i l i t y l e v e l s given are the a t t a i n e d s i g n i f i c a n c e l e v e l s f o r the four d i a g o n a l elements assuming row and column e f f e c t s ; f o r the o v e r a l l t e s t , P=0.0041. grass a s s o c i a t e L1 L2 L3 L4 T1 3 , .32 2. .25 1 , .00 0.. 94 c l o v e r T2 2, .95 3, .25 1 . .73 0. .99 producer T3 1 , .68 1 , .43 3 .38 1 . .39 T4 2. . 1 5 1 , .07 1 .84 2. .97 P1=0.2102, P2=0.1301, P3=0.0074, P4=0.0092 1 73 TABLE 1 8 . Y i e l d s (g) of phytometers of Lolium perenne (Lx), each c o l l e c t e d with a n a t u r a l neighbouring genet of T r i f o l i u m repens (Tx) from four s i t e s i n the 1939 pasture and p l a n t e d i n a l l combinations of c l o v e r type and grass type; the performances of the grass types with t h e i r n a t u r a l neighbouring c l o v e r types are shown on the p r i n c i p a l d i a g o n a l ; a l l v a l u e s are means of 3 r e p l i c a t e s ; the p r o b a b i l i t y l e v e l s given are the a t t a i n e d s i g n i f i c a n c e l e v e l s f o r the four d i a g o n a l elements assuming row and column e f f e c t s ; f o r the o v e r a l l t e s t , P=0.0126. c l o v e r a s s o c i a t e T1 T2 T3 T4 LI 6. .45 15. ,22 •23. ,01 8. .03 grass L2 7. . 1 1 .9. .59 10. , 1 3 5. .21 producer L3 1 1 , .38 6, ,53 5. ,63 8, .94 L4 12, .71 7, .92 10. ,71 7, .44 P1=0.0086, P2=0.2420, P3=0.0430, P4=0.6630 174 TABLE 19. T o t a l combined y i e l d s (g) of phytometers of Lolium perenne (L) and T r i f o l i u m repens (T) c o l l e c t e d as n a t u r a l neighbouring p a i r s from four s i t e s i n the 1939 pasture and pl a n t e d i n a l l combinations of grass type and c l o v e r type; the values f o r the n a t u r a l neighbouring p a i r s are shown on the p r i n c i p a l d i a g o n a l ; a l l values are means of 3 r e p l i c a t e s ; the p r o b a b i l i t y l e v e l s given are the a t t a i n e d s i g n i f i c a n c e l e v e l s f o r the four d i a g o n a l elements assuming row and column e f f e c t s ; f o r the o v e r a l l t e s t , P=0.0098. c l o v e r type T1 T2 T3 T4 L1 9.77 18.20 24.69 10.18 grass L2 9.69 12.84' 11.55 6.29 type L3 12.39 7.93 9.01 10.78 L4 13.65 8.92 12.10 10.41 P1=0.0125, P2=0.1346, P3=0.1133, P4=0.3307 175 TABLE 20. Combining a b i l i t y indexes f o r genet p a i r s of Lolium perenne (L) and T r i f o l i u m repens (T) c o l l e c t e d as n a t u r a l neighbouring p a i r s from four s i t e s i n the 1939 pasture and p l a n t e d i n a l l combinations of grass type and c l o v e r type; the values f o r the n a t u r a l neighbouring p a i r s are shown on the p r i n c i p a l d i a g o n a l ; a l l values are means of 3 r e p l i c a t e s ; the p r o b a b i l i t y l e v e l s given are the a t t a i n e d s i g n i f i c a n c e l e v e l s f o r the four d i a g o n a l elements assuming no row or column e f f e c t s ; f o r the o v e r a l l t e s t , P=0.0014. c l o v e r type T1 T2 T3 T4 L1 0.52 0.19 0.1 1 ' 0.27. grass L2 0.35 0.36 0.24 0.21 type L3 0.09 0.30 0.49 0.21 L4 0.12 0.17 0.13 0.48 P1=0.0045, P2=0.1287, P3=0.0088, P4=0.0121 176 FIGURE 18. Y i e l d s of phytometers of Lolium perenne (L) and T r i f o l i u m repens (T) when grown together i n d i f f e r e n t combinations of genet types. Graphs show the y i e l d s when in combination with n a t u r a l neighbours ( s t i p p l e d bar graphs) compared to y i e l d s i n a l l other combinations i n v o l v i n g A) L1 or T1, B) L2 or T2, C) L3 or T3, D) L4 or T4. A l l v a l u e s are means of 3 r e p l i c a t e s . L YIELD (g) T YIELD (g) td L YIELD (g) T YIELD (g) 3> 1—• -F 1—• ro o o o o I ,. l _ . 1 oo -F NO • • • • o o o O 1 1 o a - o _1_ L21 — l r - tO | •v II o to, - F to o * • * • * • * • ̂  • • • • • • • L2" • • • • • • • • * * • * • * " • * • * * * * • * * • * • * * * • L2" * • • * • * » * * • " • * • * » * * • —1 ro * • • • » • • • • • * • * • * • • • • i—» i i N3 L3 — i NO tO O _1_ O - F 1—1 tO r-» O CO cn -F ro O O o o o o o | 1 — 1 — 1 1 _ L to 04 O o O 1 E X I I II O o -a II o o o 00 cn I - to —i '-1 ro - F H to Tl II O O to L YIELD (g) T YIELD (g) t—' t—• *—* •fr O 00 cn -P K> K3 OJ O O o o o o o o o o o _L_ 1 1 1 1 1 1 1 1 1 L YIELD Cg) T YIELD Cg) O ~V II O cn cn o r~ H -P r~ I K> 1 —1 1 -P 1 -P o J _ to o o o 00 cn -P • • • • o o o o . . . L _ 1 — J 1 -p H —1 ro -F r~ 1 1 -F -F —I -F "0 II o o o S3 TJ II O O -P o r—» rO • • • o o o 1 _ J L _ r - H • VM • II O o o -p 00 N3 H V -P 1 79 FIGURE 19. Combining a b i l i t y indexes and t o t a l y i e l d s f o r d i f f e r e n t genet type combinations of Lolium perenne and T r i f o l i u m repens . Graphs show the values f o r the 4 combinations of n a t u r a l neighbours ( s t i p p l e d bar graphs) compared to a l l other combinations i n v o l v i n g A) L1 or T1, B) L2 or T2, C) L3 or T3, D) L4 or T4. A l l valu e s are means of 3 r e p l i c a t e s . TOTAL YIELD (g) oo cn _i i _ CA INDEX TOTAL YIELD (g) CA INDEX O (—• -I I I I I L J 1 I L o o o o o o •XT . 1 • • 1 1 —1 oo o TOTAL YIELD (9) CA INDEX TOTAL YIELD (g) CA INDEX ro o 00 cm _j 1 » 1 NO ro NO O J L_ 00 cn j |_ J I I L J l_ 0 0 O O O *-• 1-0 -tr —1 L_ . I 1 1 182 i n v o l v i n g Lx or Tx ( F i g . 19). S i g n i f i c a n t (P<0.05) d i a g o n a l e f f e c t s were obtained f o r L1-T1, L3-T3 and L4-T4. T o t a l y i e l d s are a l s o i l l u s t r a t e d in F i g u r e 19 i n order to compare them with combining a b i l i t y indexes. No strong trends are apparent although there appears to be some negative r e l a t i o n s h i p between t o t a l y i e l d and combining a b i l i t y i n combinations i n v o l v i n g L1 or T1 ( F i g . 19a) and in combinations i n v o l v i n g L3 or T3 ( F i g . 19c). Examining the data as a whole, the four n a t u r a l neighbouring genet p a i r s had the h i g h e s t combining a b i l i t y indexes but d i d not s i g n i f i c a n t l y stand out from the other combinations i n terms of t o t a l y i e l d ( F i g . 20). DISCUSSION Three out of four genet p a i r s of Lolium and T r i f o l i u m which were sampled together as immediate neighbours i n the f i e l d had higher combining a b i l i t y than those i n which the two components came from d i f f e r e n t neighbourhoods. The three s e t s of s i g n i f i c a n t r e s u l t s are s t r e s s e d s i n c e they o f f e r experimental i n d i c a t i o n of a p o t e n t i a l l y important o r g a n i z i n g f a c t o r i n community s t r u c t u r e . Combining a b i l i t y may be thought of as the c a p a c i t y to a v o i d c o m p e t i t i v e e x c l u s i o n or as the p r o b a b i l i t y of c o e x i s t e n c e f o r one genet combination r e l a t i v e to another. The term makes no assumption as to mechanism. Combining a b i l i t y i s d e f i n e d i n t h i s study by the d i f f e r e n t i a l i n the performance of the two components in the presence of each other. Those combinations i n which the s u p e r i o r component v a s t l y o u t - y i e l d e d the i n f e r i o r component have a lower p r o b a b i l i t y of c o e x i s t e n c e , or lower combining a b i l i t y , than combinations i n which the two 183 FIGURE 20. R e l a t i o n s h i p between t o t a l y i e l d and combining a b i l i t y index f o r d i f f e r e n t genet type combinations of Lolium perenne and T r i f o l i u m repens. The values f o r the n a t u r a l l y neighbouring genet p a i r s c o l l e c t e d from the four s i t e s i n the experimental f i e l d are c i r c l e d . A l l values are means of three r e p l i c a t e s . 184 CD Q _J UJ >- _J < r - o 24 -j A 18 16 - 14 - 12 - 10 H 8 H 6 J 2 -J x L1T3 x L1T2 x LAT1 x L3T1 x L4T3 ® L2T2 x L2T3 x L3T4 x L1T4 x L2T1 x L4T2 x L3T2 x L2T4 ® L4T4 © L1T1 ® L3T3 — I - 0.1 r~ 0.2 — r - 0.3 — r — 0.4 — I — 0.5 0.6 C O M B I N I N G A B I L I T Y INDEX 185 components were more s i m i l a r i n t h e i r performance. E c o l o g i c a l combining a b i l i t y i m p l i e s a high p r o b a b i l i t y of c o e x i s t e n c e due to some degree of niche s e p a r a t i o n a f f o r d i n g some measure of escape from competition (Harper 1977b). The r e s u l t i s that combinations with more e c o l o g i c a l combining a b i l i t y (more niche s e p a r a t i o n ) have a higher t o t a l y i e l d in combination than those with l e s s e c o l o g i c a l combining a b i l i t y ( l e s s niche s e p a r a t i o n ) (e.g. Seaton & Antonovics 1967, A l l a r d & Adams 1969, Remison & Snaydon - in Snaydon 1978, Joy & L a i t i n e n 1980). The most noteworthy f e a t u r e of the present r e s u l t s i s t h a t , as concerns the u n d e r l y i n g mechanism f o r higher combining a b i l i t y between n a t u r a l l y neighbouring genets compared to non- neighbouring genets, an i n t e r p r e t a t i o n of e c o l o g i c a l combining a b i l i t y i s not j u s t i f i e d . There was no i n d i c a t i o n that combinations with the h i g h e s t combining a b i l i t y had the highest t o t a l y i e l d ( F i g . 20). Higher combining a b i l i t y of n a t u r a l neighbouring genets i s i n s t e a d a t t r i b u t e d to the f a c t that the performance of each c l o v e r genet was h i g h e s t with i t s n a t u r a l neighbouring grass genet (which p a r a l l e l s the f i n d i n g s of Turkington & Harper (1979c) with res p e c t t o . c l o v e r response to n a t u r a l neighbouring grass s p e c i e s as opposed to genets), but that the performance of each grass genet was lowest with i t s n a t u r a l neighbouring c l o v e r genet. The r e s u l t i s that the d i f f e r e n t i a l i n performance of the highest y i e l d i n g component (which was always the grass) and the lowest y i e l d i n g component (which was always the c l o v e r ) was l e a s t i n genet p a i r s from the same neighbourhood. The consequence of t h i s i s a higher combining a b i l i t y of these genet p a i r s but not a higher t o t a l 186 y i e l d . Higher combining a b i l i t y between n a t u r a l neighbours in these r e s u l t s cannot be i n t e r p r e t e d i n terms of any s h i f t s i n amount of niche o v e r l a p . The i n t e r p r e t a t i o n i n s t e a d appears to i n v o l v e a form of s e l e c t i o n which reduces the d i f f e r e n t i a l i n r e l a t i v e c o m p e t i t i v e power f o r resource requirements that the two s p e c i e s have i n common. Moreover, r e l a t i v e c o m p e t i t i v e a b i l i t y appears to be genotype s p e c i f i c ; the f a c t o r s that permit high combining a b i l i t y between one p a i r of neighbouring Lolium - T r i f o l i u m genets are not the same f a c t o r s that permit a high combining a b i l i t y between a d i f f e r e n t p a i r of neighbouring Lolium - T r i f o l i u m genets from a d i f f e r e n t neighbourhood i n the community. T h i s higher combining a b i l i t y f o r n a t u r a l neighbours may be a r e s u l t of s e v e r a l f a c t o r s most l i k e l y o p e r a t i n g v a r i o u s l y i n concert r a t h e r than s e p a r a t e l y : a) P i f f e r e n t i a l f i t n e s s through l o c a l c o m p e t i t i v e a b i l i t y and recombinat ion If i n i t i a l l y the grass i s g e n e r a l l y more a g g r e s s i v e than the c l o v e r , i t may e l i m i n a t e s e v e r a l c l o v e r genotypes from the community. Within a l o c a l neighbourhood however, a few c l o v e r genotypes may possess c h a l l e n g i n g c o m p e t i t i v e a b i l i t y a g ainst the p a r t i c u l a r r e s i d e n t grass genotypes. The most c h a l l e n g i n g of such c l o v e r genotypes would leave the most descendents and i n c r e a s e i n l o c a l frequency. The exact course of s e l e c t i o n would depend on the type of competition imposed by p a r t i c u l a r grass genotypes. (The grass p o p u l a t i o n may not respond i n l i k e manner f o r reasons o u t l i n e d i n c) below). Grass genotypes would t h e r e f o r e face the most s t r i n g e n t c o m p e t i t i o n from the l o c a l l y 187 adapted c l o v e r genets that belong to the same neighbourhood and hence perform more po o r l y i n t h e i r presence than with a l i e n c l o v e r genets from other neighbourhoods. The converse would be true f o r c l o v e r genotypes. The pr o d u c t i o n of g e n e t i c recombinants would f a c i l i t a t e t r a c k i n g of l o c a l changes i n the com p e t i t i v e environment and be important to i n d i v i d u a l f i t n e s s . b) P i f f e r e n t i a l f i t n e s s through m i g r a t i o n a b i l i t y Some c l o v e r genotypes may have a gr e a t e r c a p a c i t y than others to migrate (through s t o l o n extension) from neighbourhoods imposing s t r i n g e n t c o m p e t i t i o n , and hence escape from impending m o r t a l i t y . T h i s wandering phenotype of T\_ repens permits i t to 'seek out' neighbourhoods of grass genotypes i n which i t i s compatible (Chapter 3). These may be neighbourhoods i n which competition i s avoided through niche d i f f e r e n t i a t i o n . The present evidence however suggests i n s t e a d that they are neighbourhoods i n which no member imposes an overbearing t h r e a t on the r e l a t i v e f i t n e s s of others through any s u p e r i o r a b i l i t y to e x p l o i t c o n t e s t e d resources (as i n (a) above). T h i s course of s e l e c t i o n i s e s p e c i a l l y l i k e l y i f .niche requirements are not r e a d i l y p a r t i t i o n a b l e . In the face of an a d v e r s e l y a l t e r e d c o m p e t i t i v e environment, f i t n e s s i s enhanced once again by a pr o p e n s i t y of the genotype to migrate through l a t e r a l spread. c) Beneficence between grass and c l o v e r A b e n e f i c i a l e f f e c t on the grass may be pro v i d e d by the n i t r o g e n r i c h environment i n the presence of the legume. T h i s c o u l d be a s u f f i c i e n t l y strong s e l e c t i v e f o r c e to favour those genotypes of grass which do not exert h i g h l y s t r i n g e n t c o m p e t i t i v e p r e s s u r e s on the c l o v e r . Grass genets which 188 u l t i m a t e l y leave the most descendents in l o c a l neighborhoods may t h e r e f o r e be those that are c o m p e t i t i v e enough to a v o i d c o m p e t i t i v e e x c l u s i o n by the c l o v e r but in t u r n , are not so c o m p e t i t i v e as to s e v e r e l y suppress or e l i m i n a t e the source of n i t r o g e n made a v a i l a b l e by a neighbouring c l o v e r . T h i s may be e s p e c i a l l y important to f i t n e s s i f a v a i l a b l e s o i l n i t r o g e n i s s c a r c e . The g e n o t y p e - s p e c i f i c c o a d a p t a t i o n i n l o c a l neighbourhoods may be f u r t h e r mediated by a complex i n d i r e c t mutualism i n v o l v i n g i n t e r a c t i o n of p a r t i c u l a r grass genotypes and p a r t i c u l a r s t r a i n s of symbiotic Rhizobium in the c l o v e r ( c . f . H i l l 1977). A beneficence of the above s o r t may p l a c e c o n s t r a i n t s on niche divergence. Niche divergence may not c o n t r i b u t e to higher f i t n e s s i f the m u t u a l i s t i c or commensalistic r e l a t i o n s h i p i s l o s t i n the process. S e l e c t i o n may i n s t e a d r e s u l t in an e q u i l i b r a t i o n of the r e c i p r o c a l a g g r e s s i v e n e s s of the two competing s p e c i e s e x p l o i t i n g a broadly o v e r l a p p i n g n i c h e . L o c a l s e l e c t i o n i n the present Lolium and T r i f o l i u m p o p u l a t i o n s may be o p e r a t i n g to synchronously curb and.' v i t a l i z e e f f o r t s r e s p e c t i v e l y i n the s t r u g g l e to pre-empt environmental resources, and may even i n v o l v e niche convergence as evidence suggests from the replacement s e r i e s r e s u l t s (Chapter 5). Grime (1979) a l s o advocates a phenomenon of s e l e c t i o n f o r reduced c o m p e t i t i v e vigour i n h i s theory of S - s e l e c t i o n ( " s t r e s s - t o l e r a n t " s t r a t e g y ) . S - s e l e c t i o n presumably b r i n g s about r e d u c t i o n i n both v e g e t a t i v e and r e p r o d u c t i v e v i g o u r , a d a p t a t i o n s which allow endurance of c o n t i n u o u s l y unproductive environments. 189 The r e l a t i v e c o n t r i b u t i o n of a ) , b) and c) i n developing the l o c a l b i o t i c s p e c i a l i z a t i o n r e p o r t e d here i s a t o p i c worthy of f u r t h e r i n q u i r y . The p o s s i b l e r o l e of beneficence i n d e f i n i n g s e l e c t i o n a l f o r c e s i s p a r t i c u l a r l y i n t e r e s t i n g . E c o l o g i s t s have tended to 'pigeonhole' s p e c i e s i n t e r a c t i o n s i n t o separate types, e.g. p a r a s i t i s m , p r e d a t i o n , commensalism, amensalism, co m p e t i t i o n , mutualism. Competition and beneficence have g e n e r a l l y been viewed as c o n f l i c t i n g extremes of organism i n t e r a c t i o n and before about 1960 accounts of the e v o l u t i o n a r y process l a r g e l y d i s m i s s e d c o o p e r a t i v e phenomena as not r e q u i r i n g s p e c i a l a t t e n t i o n (Axelrod & Hamilton 1981). Yet i t i s reasonable that two organisms which are competing in one r e s p e c t , may be c o o p e r a t i n g i n another. E m p i r i c a l and t h e o r e t i c a l c o n s i d e r a t i o n s , to date have concerned mostly simultaneous i n t r a - s p e c i f i c c o m petition and c o o p e r a t i o n , e.g. in D r o s o p h i l a (Mather 1961) and i n s e s s i l e i n v e r t e b r a t e s (Buss 1981). P o s s i b l e b e n e f i c i a l f u n c t i o n s i n animals i n c l u d e c o o p e r a t i o n between genders r e q u i r e d f o r producing o f f s p r i n g , i n c r e a s e d p r o t e c t i o n from weather, reduced r i s k of predation.., i n c r e a s e d f e e d i n g e f f i c i e n c y , and i n c r e a s e d i n t e r s p e c i f i c i n t e r f e r e n c e a b i l i t y (Buss 1981). Mather (1961) argues that i n so f a r as the c o o p e r a t i v e f u n c t i o n i s e s s e n t i a l to f i t n e s s , a c e r t a i n minimal c o n t r i b u t i o n to i t may be favoured even at the r i s k of r e d u c t i o n i n c o m p e t i t i v e c a p a c i t y . Evidence fo r concomitant p o s i t i v e and negative i n t e r a c t i o n s between s p e c i e s i s s c a r c e . Thomson (1978, 1980) has d e s c r i b e d cases where two s p e c i e s of entomophilous annual p l a n t s compete fo r the same p o l l i n a t o r s , but where they grow together, 1 90 p o l l i n a t i o n r a t e s f o r both are higher than when they grow alone because the more s p a t i a l l y c o n c e n t r a t e d bloom a t t r a c t s p r o p o r t i o n a t e l y more v i s i t i n g p o l l i n a t o r s . A t s a t t & 0'Dowd (1976) c i t e numerous examples from the l i t e r a t u r e suggesting that the defense of a p l a n t from h e r b i v o r e s and other pests may be improved in the company of s p e c i f i c neighbours o f f e r i n g some measure of p r o t e c t i o n and with whom some measure of c o m p e t i t i o n can be reasonably assumed. For example, P h i l l i p s & P f e i f f e r (1958) demonstrated that the grasses A g r o s t i s and Festuca gain c o n s i d e r a b l e p r o t e c t i o n from c a t t l e when a s s o c i a t e d with the noxious b u t t e r c u p Ranunculus bulbosus . Using the same s p e c i e s as in the present study, R a d c l i f f e (1972 - c i t e d i n A t s a t t & O'Dowd 1976) showed that s u s c e p t i b i l i t y of T r i f o l i u m repens to damage from grass grubs and c a t e r p i l l a r s was decreased i n mixed stands with Lolium perenne compared to i n pure stands. A f u r t h e r example i s given i n a study of the e f f e c t of removal of d i f f e r e n t components of the neighbouring v e g e t a t i o n on the p o p u l a t i o n s i z e of Rumex acetosa (Putwain & Harper 1970). I t was found that i f both d i c o t y l e d o n s and grasses were removed, i t s p o p u l a t i o n i n c r e a s e d l e s s than i f grasses alone were removed. T h i s suggests that i n some way the presence of the other d i c o t y l e d o n o u s members of the community extends the r e a l i z e d niche of Rumex acetosa . Harper (1964) emphasized, "A component removed from a h a b i t a t i s not only one of many s p e c i e s making demands on l i m i t i n g l i g h t , water, and n u t r i e n t s u p p l i e s ; i t i s a l s o a p o s s i b l e source of micro-environmental c o n d i t i o n s necessary f o r a s s o c i a t e d s p e c i e s . " Because of the high n i t r o g e n requirement of many grasses 191 combined with the n i t r o g e n - r i c h environment p r o v i d e d by the legume, a n a t u r a l b e n e f i c i a l i n t e r a c t i o n may occur together with a c o m p e t i t i v e one i n p a s t u r e s . Often, grasses have been found to be more c o m p e t i t i v e than white c l o v e r f o r other important n u t r i e n t s such as potassium (see review by Robson & Loneragen 1978). T h i s may r e s u l t i n what has been d e s c r i b e d as the " c y c l e of good and poor c l o v e r " (Blaser & Brady 1950). During 'good' c l o v e r years the s o i l n i t r o g e n i s i n c r e a s e d ; hence, grasses s t a r t growing very e a r l y i n the s p r i n g of the subsequent year. The r e s u l t a n t c o m p e t i t i o n f o r potassium may l i m i t the growth of the leguminous a s s o c i a t e (hence a 'poor' c l o v e r y e a r ) . In the f o l l o w i n g year, c l o v e r a t t a i n s more vigorous growth because the grasses face a low n i t r o g e n supply and cannot compete as a g g r e s s i v e l y f o r potassium; so the c y c l e c o n t i n u e s . A s i m i l a r phenomenon has been d e s c r i b e d in the more d e t a i l e d " g r a s s l a n d c y c l e " of Turkington & Harper (1979a). I t i s reasonable to co n s i d e r that s e l e c t i o n should favour those i n d i v i d u a l s i n both these f l u c t u a t i n g p o p u l a t i o n s that c o u l d e s t a b l i s h a more 'balancing' c o m p e t i t i v e r e l a t i o n s h i p . As i t i s p o s s i b l e that s e l e c t i o n f o r niche d i f f e r e n t i a t i o n with r e s p e c t to potassium u t i l i z a t i o n c o u l d suppress the beneficence regarding n i t r o g e n supply to the grass, s e l e c t i o n may i n s t e a d favour a decrease i n co m p e t i t i v e a b i l i t y f o r potassium i n the grass accompanied by an in c r e a s e in t h i s corresponding c o m p e t i t i v e a b i l i t y i n the legume. Those genotypes of grass which are l e s s a g g r e s s i v e i n t h i s regard should be able to s u r v i v e and reproduce more s u c c e s s i v e l y through both good and poor c l o v e r years along with t h e i r compatible c l o v e r neighbours and thereby may leave more 192 descendents. As t h i s s e l e c t i o n continues during community e v o l u t i o n , these g r a s s l a n d c y c l e s might be expected to d i m i n i s h i n amplitude or frequency. Evidence f o r such a c y c l e i n the present study can be seen i n F i g u r e 2 (Chapter 2). repens i n a l l three pastures had a higher percentage cover i n the 1979 surveys than i n the 1981 surveys. In c o n t r a s t , perenne had a higher percentage cover in the three pastures i n the 1981 surveys than i n the 1979 surveys. The c y c l e seems b a r e l y to have s t a r t e d i n the 1977 pasture, i s most pronounced in the 1958 pasture and tapers o f f somewhat i n the 1939 p a s t u r e . Poa compressa a l s o showed t h i s opposite cover t r e n d to c l o v e r i n the 1977 pasture and Holcus l a n a t u s d i s p l a y e d the same t r e n d i n the 1958 p a s t u r e . B i o t i c s p e c i a l i z a t i o n at the genotype l e v e l as demonstrated in the present study c a l l s for an organism - centered view of community s t r u c t u r e and e v o l u t i o n (Chapter 3 ) . " The t r a d i t i o n a l view d e f i n e s a community as the c o l l e c t i o n of p o p u l a t i o n s occupying a . given area, u s u a l l y thought to a f f e c t the d i s t r i b u t i o n and abundance of one another (Odum 1971). A more r e f i n e d approach regards the community as a 'montage' of e v o l v i n g neighbourhoods which c e n t e r s on the organism as the p i v o t a l u n i t of i n t e r a c t i o n (eg. MacMahon et a_l 1978, MacMahon et a l . 1981). T h i s view i s e s p e c i a l l y s u i t e d to s e s s i l e organisms such as p l a n t s s i n c e any given i n d i v i d u a l w i l l i n t e r a c t with others i n only a very l o c a l neighbourhood. The present r e s u l t s c o r r o b o r a t e the f i n d i n g s r e p o r t e d e a r l i e r (Chapters 4 and 5) that e v o l u t i o n a r y changes in the combining 193 a b i l i t y of s p e c i e s may be a consequence of not only niche d i f f e r e n t i a t i o n , but a l s o a 'balancing' of c o m p e t i t i v e a b i l i t i e s . The present study i n d i c a t e s t h a t , given s u f f i c i e n t g e n e t i c v a r i a t i o n , m i c r o - e v o l u t i o n a r y f o r c e s may be so p r e c i s e that the p r o p e r t i e s which determine r e c i p r o c a l a d a p t a t i o n i n l o c a l neighbourhoods may be neighbour s p e c i f i c even at the s c a l e of d i f f e r e n t genotypes of the same s p e c i e s . The i n t e r p r e t a t i o n exposes the need f o r more i n v e s t i g a t i o n s of the occurrence and r o l e of s p e c i e s i n t e r a c t i o n s which i n t e g r a t e beneficence and o p p o s i t i o n . 1 94 CHAPTER 7 GENERAL DISCUSSION 195 PROSPECTUS The r e s u l t s and i n t e r p r e t a t i o n s of the work presented i n t h i s t h e s i s c a l l f o r a general d i s c u s s i o n of the phenomenon of c o e x i s t e n c e i n systems of competition between p l a n t s . In Chapter 1, the l a r g e l y overlooked d i s t i n c t i o n between the concepts of fundamental niche and c o m p e t i t i v e a b i l i t y i s e s t a b l i s h e d and a comprehensive p e r s p e c t i v e f o r the meaning of the l a t t e r i s o f f e r e d . T h i s enables the t h e s i s to proceed with minimal ambiguity i n the p a r l a n c e of c o m p e t i t i o n and c o e x i s t e n c e . In Chapter 2, o r d i n a t i o n of t i m e - s e r i e s percentage cover surveys ( F i g . 4) r e v e a l e d trends which support the suggestion that the three pastures under study represent d i f f e r e n t stages .in a common p r o g r e s s i o n of community development. T h i s i s f u r t h e r c o r r o b o r a t e d by t i m e - s e r i e s surveys of c o n t a c t sampling presented in Chapter 3 ( F i g . 8). Contacts between i n d i v i d u a l s are i n t e r p r e t e d as r e f l e c t i n g an o p e r a t i o n a l measure of s p e c i e s i n t e r a c t i o n . The dynamics of i n t e r s p e c i f i c . c o n t a c t s t h e r e f o r e represent p a t t e r n s 'of a s s o c i a t i o n and c o e x i s t e n c e at the f i n e - s c a l e l e v e l of i n d i v i d u a l experience. A trend of i n c r e a s i n g s t a b i l i t y of i n t e r - s p e c i f i c a s s o c i a t i o n i n o l d e r pastures was d i s c o v e r e d and t h i s i s used as a b a s i s f o r c o n s t r u c t i n g a ' p o r t r a i t ' of pasture community e v o l u t i o n which a t t r i b u t e s within-community temporal changes to the s e l e c t i v e f o r c e s a c c r u i n g from b i o t i c i n t e r a c t i o n s ( F i g . 9). Armed with t h i s i n f o r m a t i o n , the stage was set f o r a d d r e s s i n g the c e n t r a l i s s u e of t h i s t h e s i s : What mechanisms are r e s p o n s i b l e f o r t h i s ' s e t t l i n g down' of community 196 dynamics and p e r m i t t i n g c o e x i s t e n c e i n these systems? It i s t a c i t l y assumed that p r e d a t i o n ( i . e . grazing) and other d i s t u r b a n c e s are important f a c t o r s p e r m i t t i n g d i v e r s i t y i n pastures through t h e i r e f f e c t s i n suppressing p o t e n t i a l dominants (e.g. Harper 1969, Watkin & Clements 1978, Grime 1979). It i s a l s o a x i o m a t i c i n ecology that a phenomenon as broad as c o e x i s t e n c e w i l l r a r e l y be f u l l y e x p l a i n e d by a s i n g l e f a c t o r . The present work t h e r e f o r e focuses on the hypothesis that the b i o t i c f a c t o r of neighbour i n t e r a c t i o n s , e s p e c i a l l y c ompetition i s important. With d i f f e r e n t stages of community development a v a i l a b l e at' the same time, an o p p o r t u n i t y was a v a i l a b l e f o r studying p r e c i s e l y how the c o m p e t i t i v e r e l a t i o n s h i p between a p a r t i c u l a r p a i r of s p e c i e s changes d u r i n g community e v o l u t i o n . N a t u r a l s e l e c t i o n l e a d i n g to c o e x i s t e n c e i n contexts of competition and other neighbour i n t e r a c t i o n s may be d e f i n e d as s e l e c t i o n f o r 'combining a b i l i t y ' . The three experimental designs f o r s t u d y i n g c o m p e t i t i v e i n t e r a c t i o n s (Chapters 4, 5 and 6) . y i e l d e d c o r r o b o r a t i v e data. Evidence. i n d i c a t e s that combining a b i l i t y may be of two c o n t r a s t i n g types: 1) ' e c o l o g i c a l ' combining a b i l i t y (niche d i f f e r e n t i a t i o n ) , and 2) 'competitive' combining a b i l i t y (balanced c o m p e t i t i v e a b i l i t i e s ) . The r a m i f i c a t i o n s of these f i n d i n g s to contemporary c o e x i s t e n c e theory are pursued here i n r e l a t i o n to systems i n which competition i s an important f o r c e of n a t u r a l s e l e c t i o n . The arguments presented f o l l o w from r e c o g n i z i n g a c l e a r d i s t i n c t i o n between the concepts of 'niche' and 'competitive a b i l i t y ' (Chapter 1). 1 97 COMPONENTS OF A GENERALIZED COEXISTENCE THEORY A theory f o r the c o e x i s t e n c e of p l a n t competitors i s set out below which i n t e g r a t e s t r a d i t i o n a l theory with a broader i n t e r p r e t a t i o n of the o p e r a t i o n of n a t u r a l s e l e c t i o n than p r e v i o u s l y r e c o g n i z e d . I t s e v o l u t i o n a r y premise i n c o r p o r a t e s the dynamics and v a r i a b i l i t y of two primary components; 1) fundamental niche requirements, and 2) r e l a t i v e c o m p e t i t i v e a b i l i t y . A q u a l i t a t i v e model i s p o r t r a y e d i n F i g u r e 24 and i s d e s c r i b e d by three u n d e r l y i n g concepts. A) Gaussian c o e x i s t e n c e The t h e o r e t i c a l framework of Gause's p r i n c i p l e i s denoted by the contents of the dashed box i n F i g u r e 21 which i s i n re f e r e n c e to a h y p o t h e t i c a l two-species system. In regard to the f i r s t t h e o r e t i c a l component, i f fundamental niche requirements are s u f f i c i e n t l y s i m i l a r ( i n the sense of ' l i m i t i n g s i m i l a r i t y ' (MacArthur & Lev i n s 1967)), Gause's p r i n c i p l e pronounces a r e l e n t l e s s proceeding toward c o m p e t i t i v e e x c l u s i o n of one of the s p e c i e s . Given t h i s c o n d i t i o n f o r the f i r s t component, (although never part of any formal statement) the p r i n c i p l e i m p l i c i t l y invokes an assumption f o r the second component - i . e . r e l a t i v e c o m p e t i t i v e a b i l i t i e s w i l l be s u f f i c i e n t l y d i f f e r e n t . T h i s d i f f e r e n c e i n com p e t i t i v e a b i l i t i e s i s assumed to be i n v a r i a b l y present between any two i n t e r a c t i n g s p e c i e s u n t i l e x c l u s i o n of one s p e c i e s i s complete. Gaussian c o e x i s t e n c e (a c o r o l l a r y of the co m p e t i t i v e e x c l u s i o n p r i n c i p l e ) i s p o s s i b l e only i f fundamental niche requirements 198 FIGURE 21. The schematic s t r u c t u r e of a general theory of s p e c i e s c o e x i s t e n c e i n contexts where competition i s an important f o r c e of n a t u r a l s e l e c t i o n . The general theory recognizes two b a s i c and d i s t i n c t components: (1) Fundamental niche requirements, and (2) R e l a t i v e c o m p e t i t i v e a b i l i t i e s . Three concepts are represented and d i s c u s s e d i n the t e x t : (A) t r a d i t i o n a l theory f o r 'Gaussian' c o e x i s t e n c e (dashed box); and two e v o l u t i o n a r y mechanisms which permit c o e x i s t e n c e - i ) s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y (niche d i f f e r e n t i a t i o n ) (B), and i i ) s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y ( c o e v o l u t i o n ) (C). r -a ( B ) SELECTION FOR ECOLOGICAL COMBINING ABILITY reduces fundamental niche overlap (D FUNDAMENTAL NICHE REQUIREMENTS s u f f i c i e n t l y s i m i l a r (2) RELATIVE COMPETITIVE ABILITIES s u f f i c i e n t l y d i f ferent (A) - i ( i ) FUNDAMENTAL NICHE REQUIREMENTS s u f f i c i e n t l y d i f f e r e n t (2) (compet i t ive i n teract ion absent in exc lus ive n iches) J (C) SELECTION FOR COMPETITIVE COMBINING ABILITY reduces difference in relative corq)etitive abilities 200 are d i f f e r e n t 'enough' ( i . e . a ' l i m i t e d ' amount of o v e r l a p i s p e r m i t t e d ) , the c r u c i a l consequence being that the two s p e c i e s do not always have to compete because each has some e x c l u s i v e niche space, e i t h e r s p a t i a l l y or temporally. A c c o r d i n g to the Gaussian view, where competition occurs between two s p e c i e s i n respect to a broadly o v e r l a p p i n g niche, the c o m p e t i t i v e e x c l u s i o n of one i s impending. That the Gaussian philosophy i s s t i l l p e r v a s i v e in c u r r e n t e c o l o g i c a l t h i n k i n g i s evident by a recent quote from Newman (1982): " I t i s g e n e r a l l y accepted that i n order to c o e x i s t more than t r a n s i e n t l y s p e c i e s must d i f f e r - they must show niche s e p a r a t i o n . If s p e c i e s are too s i m i l a r a l l but one w i l l be e l i m i n a t e d i n c o m p e t i t i o n " . I t i s not d i f f i c u l t to imagine how such profound statements a s s o c i a t e d with Gause's p r i n c i p l e c o u l d have been a product of the e a r l y e v o l u t i o n i s t s ' c e n t r a l focus on such ideas as ' s t r u g g l e f o r e x i s t e n c e ' and ' s u r v i v a l of the f i t t e s t ' . Such were strong outward themes of Darwin and Wallace conceived from t h e i r awareness of the inherent c a p a c i t y of p o p u l a t i o n s to i n c r e a s e i n s i z e . T h i s was f u r t h e r r e i n f o r c e d by e a r l y experimental s t u d i e s demonstrating c o m p e t i t i v e e x c l u s i o n i n l a b o r a t o r y p o p u l a t i o n s (Gause 1934, Park 1948, 1954). What i s s u r p r i s i n g i s that t h i s same theme p e r s i s t s l a r g e l y u n a l t e r e d today. As Sheppard (1975) has p o i n t e d out, the use of the word ' s t r u g g l e ' which suggests p h y s i c a l combat, has obscured the " a l l - e m b r a c i n g nature of s e l e c t i o n " and the importance of (and p o t e n t i a l f o r ) the immense v a r i a t i o n w i t h i n s p e c i e s that Darwin was r e a l l y d r i v i n g a t . 201 B) Coexistence by s e l e c t ion f o r e c o l o g i c a l combining a b i l i t y (niche d i f f e r e n t i a t i o n ) Upon r e c o g n i z i n g that a p o p u l a t i o n w i l l u s u a l l y have g e n e t i c v a r i a t i o n p e r t a i n i n g to fundamental niche requirements, e c o l o g i s t s turned t h e i r a t t e n t i o n from i n v e s t i g a t i o n s of what enables one s p e c i e s to win i n a c o m p e t i t i v e i n t e r a c t i o n ( c ompetitive e x c l u s i o n ) (e.g. Sakai & Gotoh 1955, Lerner & Ho 1961, Mather & Cooke 1962, Gale 1964), to i n v e s t i g a t i n g the r o l e of n a t u r a l s e l e c t i o n i n the e v o l u t i o n of niche d i f f e r e n t i a t i o n and hence p e r m i t t i n g c o e x i s t e n c e ( F i g . 21-B). In p l a n t i n t e r a c t i o n s t h i s has been termed s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y (Harper 1964, 1967, 1977b). Under t h i s theory, s p e c i e s c o e x i s t because e v o l u t i o n , by the s e l e c t i o n p r e s s u r e s of c o m p e t i t i o n , r e s u l t s i n avoidance of a c o m p e t i t i v e i n t e r a c t i o n through niche d i f f e r e n t i a t i o n . N o t ice that t h i s e v o l u t i o n a r y approach to e x p l a i n i n g c o e x i s t e n c e i s s t i l l entrenched w i t h i n a 'Gaussian' framework (dashed box i n F i g . 21). I t s t i l l assumes t h a t : 1) c o e x i s t e n c e i s only achieved i f niche requirements become s u f f i c i e n t l y d i f f e r e n t and hence i n t e r s p e c i f i c c o m petition i s r e l a x e d (or e l i m i n a t e d ) and 2) where niche o v e r l a p remains, c o m p e t i t i v e e x c l u s i o n w i l l ensue. In essence then, s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y i s an extension or c o r o l l a r y of Gause's p r i n c i p l e ( c . f . Grubb 1977, p.107) and presupposes that under high i n t e r - s p e c i f i c d e n s i t y , n a t u r a l s e l e c t i o n may r e s u l t in niche d i f f e r e n t i a t i o n and hence c o e x i s t e n c e - e l s e , c o m p e t i t i v e e x c l u s i o n i s forthcoming. If each s p e c i e s i s a f f e c t e d more by the presence of the 202 other than by the presence of i n t r a s p e c i f i c i n d i v i d u a l s , then c o m p e t i t i o n i s of the ' i n t e r f e r e n c e ' type (Harper 1977b). One s p e c i e s may be more a g g r e s s i v e than the other, but d i r e c t i o n a l s e l e c t i o n may be strong i n both s p e c i e s and d i f f e r e n c e s i n r e l a t i v e f i t n e s s amongst i n d i v i d u a l s ( i n each s p e c i e s ) occupying d i f f e r e n t regions of niche space may r e s u l t i n niche divergence. In the more common ' e x p l o i t a t i o n ' type of com p e t i t i o n i n p l a n t s however, i f co m p e t i t i v e a b i l i t i e s are unequal, the weaker competitor i s g e n e r a l l y a f f e c t e d more by the presence of i n t e r s p e c i f i c i n d i v i d u a l s and the stronger competitor by the presence of i n t r a s p e c i f i c i n d i v i d u a l s (Harper 1977b). The g r e a t e s t s e l e c t i o n p r essure here w i l l be exe r t e d on a weak competitor when faced with a much stronger competitor. I f the d i f f e r e n c e i n the s p e c i e s ' r e l a t i v e c o m p e t i t i v e a b i l i t i e s i s l a r g e , d i r e c t i o n a l s e l e c t i o n w i l l be str o n g and niche s h i f t (displacement) may r e s u l t i n the weaker competitor, but there would be no reason to expect niche displacement i n the stronger competitor. S i g n i f i c a n t c o m p e t i t i v e pressure f o r the stronger competitor comes p r i m a r i l y from i n d i v i d u a l s of. i t s own s p e c i e s . From the standpoint of p r e v e n t i n g c o m p e t i t i v e e x c l u s i o n , the most important r e q u i s i t e i s that the weaker competitor secure an e x c l u s i v e niche space. The c e n t r a l f e a t u r e of t h i s mechanism i s that c o e x i s t e n c e i s p e r m i t t e d as long as co m p e t i t i v e i n t e r a c t i o n i s reduced or v i r t u a l l y e l i m i n a t e d and t h i s i s accomplished by s e l e c t i o n f o r niche d i f f e r e n t i a t i o n when competitors encounter one another r e s u l t i n g i n each having an e x c l u s i v e niche space. If comp e t i t i o n i s avoided, more e f f i c i e n t e x p l o i t a t i o n of the 203 environment r e s u l t s and two s p e c i e s w i l l have a higher t o t a l y i e l d i n combination a f t e r s e l e c t i o n than before s e l e c t i o n . Evidence f o r t h i s mechanism may be i n t e r p r e t e d f o r some s p e c i e s p a i r s i n the mixture d i a l l e l study ( F i g . 11) and i n the replacement s e r i e s a n a l y s i s f o r Holcus lanatus and T r i f o l i u m repens ( F i g . 16). S e l e c t i o n o p e r a t i n g i n t h i s manner w i l l be l i m i t e d by the s p e c i e s ' p o t e n t i a l to generate new genetic v a r i a n t s capable of pre-empting novel n i c h e s . T h i s theme has become n e a r l y axiomatic i n recent e c o l o g i c a l s t u d i e s f a r too numerous to l i s t (see f o r example Cody 1974, Grubb 1977, Diamond 1978, van den Bergh & Braakhekke 1978). Most evidence however i s c i r c u m s t a n t i a l ; some have r e c e n t l y argued that i n f a c t there is- l i t t l e e m p i r i c a l b a s i s to support the commonly h e l d view that c o e x i s t e n c e i n m u l t i - s p e c i e s g u i l d s i s a r e s u l t of niche divergence (Wiens 1977, C o n n e l l 1980). C) Coexistence by s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y ( c o e v o l u t i o n ) Genetic v a r i a t i o n w i l l be present i n a p o p u l a t i o n p e r t a i n i n g not only to p a r t i c u l a r types of fundamental niche requirements, but a l s o p e r t a i n i n g to r e l a t i v e c o m p e t i t i v e a b i l i t y f o r any common niche requirements. The same s e l e c t i o n a l f o r c e of competition w i l l operate on both of these components of g e n e t i c v a r i a t i o n . The exhaustive l i t e r a t u r e on p l a n t g e n e t i c s suggests that the number of genes c o n t r o l l i n g p l a n t c h a r a c t e r s , i s very l a r g e ( S o l b r i g 1980). Indeed, a vast number of a l l e l e s w i l l be present i n a p o p u l a t i o n which determine the v a r i o u s s t a t e s of a l l the c h a r a c t e r s i n Table 1 (Chapter 1) which govern 204 c o m p e t i t i v e a b i l i t y . P o s s i b i l i t i e s are even f u r t h e r m u l t i p l i e d in c o n s i d e r i n g that one genotype may f o r example have a gr e a t e r uptake e f f i c i e n c y f o r one n u t r i e n t while a second genotype may have a gr e a t e r uptake e f f i c i e n c y f o r a second n u t r i e n t ; which w i l l have the s e l e c t i v e advantage w i l l depend on the nature of the p r e v a i l i n g c o m p e t i t i v e p r e s s u r e s . The o p p o r t u n i t i e s f o r n a t u r a l s e l e c t i o n to improve r e l a t i v e c o m p e t i t i v e a b i l i t y i n a p l a n t p o p u l a t i o n seem innumerable. Sakai (1961) has proposed that c o m p e t i t i v e a b i l i t y i n p l a n t s can be t r e a t e d i t s e l f as a 'cha r a c t e r ' under at l e a s t p a r t i a l g e n e t i c c o n t r o l . Under t h i s mechanism f o r c o e x i s t e n c e , any genotype that i s a s u p e r i o r competitor a c t s as a s e l e c t i v e agent on i t s own comp e t i t o r s . The c r u c i a l p o i n t i s that such s e l e c t i o n pressure w i l l operate r e c i p r o c a l l y and s e q u e n t i a l l y i n . two competing p o p u l a t i o n s so any ' s u p e r i o r ' genotype u l t i m a t e l y l o s e s i t s co m p e t i t i v e advantage. ' S u p e r i o r i t y ' i n competition t h e r e f o r e a l t e r n a t e s between (and amongst) members of the two p o p u l a t i o n s as v a r i o u s c h a r a c t e r s which confer c o m p e t i t i v e a b i l i t y are generated, combined, recombined, s e l e c t e d f o r , s e l e c t e d a g a i n s t , and s e l e c t e d f o r once again. L o c a l neighbourhoods are c o n s t a n t l y engaged i n a f i n e - t u n i n g process that a l t e r s the way members respond to one another. A s p e c i e s p o p u l a t i o n t h e r e f o r e need not a v o i d competition i n order to a v o i d c o m p e t i t i v e e x c l u s i o n ; i t may i n s t e a d maintain the p o t e n t i a l to generate (or a c q u i r e , through mutation, gene flow or recombination) and propagate new g e n e t i c v a r i a n t s with i n c r e a s e d c o m p e t i t i v e power under the p r e v a i l i n g c o m p e t i t i v e p r e s s u r e s . In each such event of n a t u r a l s e l e c t i o n , once again the s t r o n g e s t s e l e c t i o n 205 pressure w i l l be exerted by the more powerful competitor on the weaker competitor. Competitive e x c l u s i o n i s avoided i f n a t u r a l s e l e c t i o n favours those g e n e t i c v a r i a n t s i n the weaker competitor that have an o v e r a l l c o m p e t i t i v e a b i l i t y at l e a s t comparable to that of the stronger competitor. If some form of beneficence a l s o occurs between competitors, s e l e c t i o n may c o n c u r r e n t l y favour those g e n e t i c v a r i a n t s i n the stronger competitor that have an o v e r a l l c o m p e t i t i v e a b i l i t y more comparable to that of the weaker competitor (Chapter 6). Coexistence under t h i s mechanism i s achieved by r e c i p r o c a l n a t u r a l s e l e c t i o n o p e r a t i n g s e q u e n t i a l l y w i t h i n both s p e c i e s , each time to reduce the d i f f e r e n c e i n r e l a t i v e c o m p e t i t i v e a b i l i t i e s which might otherwise l e a d to c o m p e t i t i v e e x c l u s i o n of the weaker competitor ( F i g . 21-C). F o l l o w i n g convention, t h i s mechanism may be termed, ' s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y ' ( c . f . e c o l o g i c a l (=niche) combining a b i l i t y ) . T o t a l y i e l d i n combination need not i n c r e a s e a f t e r s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y . T h i s i s c o n s i s t e n t with the r e s u l t s f o r 5 species combinations i n the d i a l l e l a n a l y s i s (Table 14).and in the combining a b i l i t y of n a t u r a l neighbouring genets of T r i f o l i u m repens and Lolium perenne compared to genet combinations of these two s p e c i e s from a l i e n neighbourhoods d i s c u s s e d i n Chapter 6 ( F i g . 19). It i s important not to be mislead i n the above d i s c u s s i o n by the terms 'stronger' and 'weaker' competitor. ' R e c i p r o c a l s e l e c t i o n ' under t h i s mechanism i s not an 'arms race' i n a p e r p e t u a l ' f i g h t ' implying an i n c r e a s i n g l y c o s t l y e n t e r p r i s e as one might imagine i n the context of a g g r e s s i v e i n t e r f e r e n c e i n 206 animals. In c o n t r a s t , i t i s p o s s i b l e that any one of s e v e r a l more s u b t l e c h a r a c t e r s t i c s i n p l a n t s (e.g. Table 1, Chapter 1) may a f f o r d an i n d i v i d u a l a c o m p e t i t i v e edge. Moreover, at d i f f e r e n t times, at d i f f e r e n t ages and under d i f f e r e n t types of co m p e t i t i v e p r e s s u r e s , d i f f e r e n t c h a r a c t e r s i t i c s or s u i t e s of c h a r a c t e r i s t i c s may be important i n p r o v i d i n g c o m p e t i t i v e ' s t a y i n g power'. S e l e c t i o n i s t h e r e f o r e not u n i d i r e c t i o n a l but m u l t i d i r e c t i o n a l and one need not think of a p o p u l a t i o n 'running out' of genes or c h a r a c t e r i s t i c s f o r co m p e t i t i v e a b i l i t y . Development of more balanced c o m p e t i t i v e a b i l i t i e s i s seen i n the replacement s e r i e s a n a l y s i s of Holcus lana t u s and Lolium perenne from the d i f f e r e n t aged p a s t u r e s ( F i g . 16c, Chapter 5). Any measure of s u p e r i o r i t y a c q u i r e d by one s p e c i e s a f f e c t i n g the f i t n e s s of a second s p e c i e s w i l l i n turn be t r a c k e d by ge n e t i c changes i n the second s p e c i e s . The r e s u l t i s ongoing c o e v o l u t i o n . Competitive pressure w i l l of course come from members of the same s p e c i e s as w e l l as from d i f f e r e n t s p e c i e s and s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y w i l l operate both i n t r a - and i n t e r - s p e c i f i c a l l y . In f a c t , i n t r a - s p e c i f i c c o e x i s t e n c e can be l a r g e l y i n t e r p r e t e d by t h i s p r o c e s s . S e l e c t i o n f o r i n t e r - s p e c i f i c c o m p e t i t i v e combining a b i l i t y o p e r a t i n g i n two s p e c i e s i n e f f e c t means that they c o e x i s t because they continue to behave c o m p e t i t i v e l y as e s s e n t i a l l y a ' s i n g l e s p e c i e s ' in which the average d i f f e r e n c e i n co m p e t i t i v e a b i l i t y between i n d i v i d u a l s i s comparatively small r e g a r d l e s s of s p e c i e s . Under high d e n s i t y and resource l i m i t a t i o n , such a c o l l e c t i o n of i n d i v i d u a l s would be expected to experience a process analagous 207 to ' s e l f - t h i n n i n g ' . From the p e r s p e c t i v e of the ' i n d i v i d u a l ' i n the community there i s no reason to d i f f e r e n t i a t e between i n t e r - s p e c i f i c and i n t r a - s p e c i f i c c o m p e t i t i v e combining a b i l i t y f o r s p e c i e s with broadly o v e r l a p p i n g n i c h e s . The only matter of any consequence to the i n d i v i d u a l faced with c o m p e t i t i o n with another i n d i v i d u a l i s the d i f f e r e n c e i n t h e i r r e l a t i v e c o m p e t i t i v e a b i l i t i e s r e g a r d l e s s of s p e c i e s i d e n t i t y . As p o i n t e d out e a r l i e r , s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y i m p l i e s an i n c r e a s e i n t o t a l y i e l d of a mixture. Measuring the t o t a l y i e l d of a s p e c i e s mixture i s analagous to measuring the y i e l d of a heterozygote r e s u l t i n g from a c r o s s between two homozygotes f o r a s s e s s i n g t h e i r 'genetic combining a b i l i t y ' (Harper 1964). However, i n the product of a s p e c i e s m i x t u r e , i n c o n t r a s t with the product of a g e n e t i c - c r o s s , the two components which are being assessed f o r combining a b i l i t y remain d i s t i n c t and the c o n t r i b u t i o n of each to the t o t a l y i e l d can be measured s e p a r a t e l y . Competitive combining a b i l i t y i s a s p e c i a l type of combining a b i l i t y which takes account of the v a r i a b i l i t y which may occur i n the r e l a t i v e c o n t r i b u t i o n of the two components and i s a r e f l e c t i o n of t h e i r p o t e n t i a l to c o n t r i b u t e e q u a l l y to the t o t a l y i e l d . In s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y , an i n c r e a s e i n y i e l d a l s o occurs (as with e c o l o g i c a l combining a b i l i t y ) , but only i n the i n f e r i o r component. Based on the above c o n s i d e r a t i o n s , i n t e r p r e t a t i o n s of the c o m p e t i t i v e r e l a t i o n s between s p e c i e s from the experimental i n v e s t i g a t i o n s r e p o r t e d i n t h i s t h e s i s are summarized in Table 21. 208 TABLE 21. A summary of i n t e r p r e t a t i o n s of c o m p e t i t i v e r e l a t i o n s between s p e c i e s r e p o r t e d from the experimental i n v e s t i g a t i o n s . SPECIES PAIR EXPERIMENTAL DESIGN USED FOR INVESTIGATING COMPETITIVE RELATION CHAPTER IN THIS THESIS INTERPRETATION OF THE TYPE OF COMBINING ABILITY RESULTING FROM SELECTION DACTYLIS GLOME RATA / HOLCUS LANATUS D i a l l e l if Eco log ica l DACTYLIS GLOMERATA / LOLIUM PERENNE D i a l l e l if none (Competitive Exclusion) DACTYLIS GLOMERATA / POA C6MPRESSA D i a l l e l if Compet i t i ve DACTYLIS GLOMERATA / TRIFOLIUM REPENS D i a l l e l if none (Competitive Exclusion) HOLCUS LANATUS / Dial le i if Compet i t i ve LOLIUM PERENNE Replacement Series 5 Compet i t i ve HOLCUS LANATUS / POA COMPRESSA D i a l l e l if Eco log ica l & Competitive HOLCUS LANATUS / D i a l l e l k Eco log ica l 6 Competitive TRIFOLIUM REPENS Replacement Series 5 Eco log ica l 6 Competitive LOLIUM PERENNE / POA COMPRESSA D i a l l e l if Compet i 11ve Dial l e i k Compet i 11 ve LOLIUM PERENNE / TRIFOLIUM REPENS Replacement Series 5 Compet i 11ve Reciprocal Transplant ing 6 Competitive POA COMPRESSA / TRIFOLIUM REPENS Dial l e i if Compet i t i ve 210 OTHER MECHANISMS FOR COMPETITIVE BALANCE The mechanism of s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y i m p l i e s a tenuous 'balance' between competitors f o r the same resources and that t h i s balance i s maintained by n a t u r a l s e l e c t i o n . Other mechanisms of c o e x i s t e n c e a l s o based on a 'balance' between competitors have been proposed and i t i s necessary to p o i n t out how they d i f f e r from s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y . 1) Non-evolutionary mechanisms As r e l a t i v e c o m p e t i t i v e a b i l i t y i s mediated by environmental c o n d i t i o n s , temporal v a r i a t i o n i n the environment may allow c o e x i s t e n c e by c o n t i n u a l l y a l t e r i n g (e.g. s e a s o n a l l y ) the p r e v a i l i n g r e l a t i v e c o m p e t i t i v e a b i l i t i e s of p o p u l a t i o n s i n h a b i t i n g i t , where f i r s t one s p e c i e s , then another then the f i r s t again (etc.) has the c o m p e t i t i v e advantage (Hutchinson 1948, Gulmon 1979, Fowler 1982). 'Non-equilibrium c o e x i s t e n c e ' may r e s u l t when f a c t o r s such as d i s t u r b a n c e , d i s e a s e or p r e d a t i o n serve to suppress s u p e r i o r competitors which would otherwise c o m p e t i t i v e l y exclude other s p e c i e s (Caswell 1978, Huston 1979, P i c k e t t 1980). Coexistence may a l s o be p o s s i b l e i f r e l a t i v e c o m p e t i t i v e a b i l i t i e s vary with age so that each s p e c i e s has an advantage in d i f f e r e n t phases of the l i f e c y c l e (Watt 1955, Grubb 1977, Fagerstrom & • Agren 1979). In a m u l t i s p e c i e s system c o e x i s t e n c e may be p o s s i b l e by a c i r c u l a r network of r e l a t i v e c o m p e t i t i v e a b i l i t i e s . Species A f o r example may be a s u p e r i o r competitor to s p e c i e s B while s p e c i e s 21 1 B i n turn excludes s p e c i e s C which wins i n com p e t i t i o n with s p e c i e s A (Pianka 1979). These mechanisms may be important to some extent in the pasture system i n the present study. Such e x p l a n a t i o n s however are non e v o l u t i o n a r y ; they are not concerned with the g e n e t i c a l l y dynamic and v a r i a b l e p o t e n t i a l of r e l a t i v e c o m p e t i t i v e a b i l i t y i n p o p u l a t i o n s or the r o l e of n a t u r a l s e l e c t i o n i n c o n t i n u a l l y a d j u s t i n g i t in a process of ongoing c o e v o l u t i o n . I t i s t h i s f a c t o r which s e t s apart the mechanism of s e l e c t i o n f o r co m p e t i t i v e combining a b i l i t y . 2) Frequency dependent s e l e c t i o n Models f o r frequency dependent b a l a n c i n g of competitors have a l s o been c o n s i d e r e d as p o s s i b l e mechanisms of s t a b l e c o e x i s t e n c e of competitors (e.g. Pimentel et. a_l . 1965, Ayala 1966, 1970, 1971, 1972, Levin 1969, 1971, Leon 1974, L e v i n & Udovic 1977). Pimentel (1968) argues that the r a r e r of two competing s p e c i e s w i l l be i n v o l v e d more o f t e n i n i n t e r s p e c i f i c encounters and the commoner i n i n t r a s p e c i f i c encounters. Hence, the r a r e r .species w i l l experience . stronger s e l e c t i o n f o r improvement i n i n t e r s p e c i f i c c o m petition and l i k e w i s e the commoner sp e c i e s f o r improvement i n i n t r a s p e c i f i c c o m p e t i t i o n . T h i s would t h e o r e t i c a l l y l e a d by a process of ge n e t i c feedback to e c o l o g i c a l s t a b i l i t y . A g e n e t i c b a s i s f o r such r e v e r s a l s i n s p e c i e s dominance may e x i s t when at l e a s t at one of the l o c i c o n t r o l l i n g c o m p e t i t i v e performance ( i n each s p e c i e s ) , one of the homozygotes has maximum i n t e r s p e c i f i c c o m p e t i t i v e a b i l i t y but minimum i n t r a s p e c i f i c f i t n e s s , and v i c e versa f o r the other homozygote (Ayala 1970). 212 There are important assumptions b u i l t i n t o t h i s frequency- dependent model that are not found in the mechanism of s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y . In the former, two po p u l a t i o n s are p u r p o r t e d l y maintained due to a dampening of wide o s c i l l a t i o n s of sp e c i e s dominance. The only evidence f o r such o s c i l l a t i o n s have come from l a b o r a t o r y p o p u l a t i o n s of animals; i n nature such o s c i l l a t i o n s have never been observed i n s e s s i l e higher p l a n t s (Antonovics 1978). Moreover, as t h i s mechanism makes no s t i p u l a t i o n r e garding the degree of niche o v e r l a p , i t i m p l i e s that s e l e c t i o n f o r i n t r a s p e c i f i c c o m p e t i t i v e a b i l i t y may be in o p p o s i t i o n to s e l e c t i o n f o r i n t e r s p e c i f i c c o m p e t i t i v e a b i l i t y i n respect to the same co n t e s t e d r e s o u r c e s . I t i m p l i e s that a t t r i b u t e s which make an i n d i v i d u a l a good i n t e r s p e c i f i c competitor are d i f f e r e n t from those a t t r i b u t e s that make i t a good i n t r a s p e c i f i c competitor and that t h e i r e f f e c t s are i n f a c t c o n f l i c t i n g . To suggest that t h i s i s common in p l a n t s i s unfounded. In a p l a n t s p e c i e s which i s a good i n t r a - s p e c i f i c competitor but experiences high i n t e r - s p e c i f i c d e n s i t y , s e l e c t i o n in the p o p u l a t i o n may indeed r e s u l t . i n improved i n t e r - s p e c i f i c c o m p e t i t i v e a b i l i t y ,• but. there i s no apparent reason that i t should n e c e s s a r i l y a l s o r e s u l t i n reduced i n t r a - s p e c i f i c c o m p e t i t i v e a b i l i t y . If an i n d i v i d u a l has a r a p i d uptake rate f o r n u t r i e n t s which are i n l i m i t e d supply f o r example, t h i s w i l l c o nfer a s u p e r i o r c o m p e t i t i v e a b i l i t y over i n d i v i d u a l s with a slower uptake r a t e , r e g a r d l e s s of s p e c i e s . Lawlor and Maynard Smith (1976) argue that such models are more d i r e c t l y a p p l i c a b l e to cases i n which com p e t i t i o n i s mediated by b e h a v i o u r a l i n t e r f e r e n c e (as i n many 213 animals) r a t h e r than merely r e l a t i v e e x p l o i t a t i o n of resources as i s more common i n p l a n t s . S e l e c t i o n f o r co m p e t i t i v e a b i l i t y here i n v o l v e s ' a - s e l e c t i o n ' d i s c u s s e d by G i l l (1974) which "... r e f e r s e x p l i c i t l y to the a c q u i s i t i o n of i n t e r f e r e n c e phenomena, mechanisms which prevent a competitor from g a i n i n g access to the resources ...". I t i s t h e r e f o r e e s p e c i a l l y d i f f i c u l t with p l a n t s to imagine an instance where Pimentel's g e n e t i c feedback mechanism f o r competitors should be important i n nature with the p o s s i b l e exception of a u t o - t o x i c i t y by a l l e l o p a t h y (e.g. Webb et a l . 1967). That such a mechanism should be important at a l l i n nature i s viewed u n l i k e l y by some (Levin 1971). In sta r k c o n t r a s t to the above p e r s p e c t i v e f o c u s s i n g on animals, frequency dependent c o m p e t i t i v e phenomena in p l a n t s i s u s u a l l y c o n s i d e r e d a consequence of the f a c t that two s p e c i e s escape some measure of competition with each other ( i . e . they have niche d i f f e r e n c e s ) (Harper 1977b). (See Fi g u r e 12 and theory presented i n Chapter 5). T h i s i s e v i d e n t , f o r example, from frequency dependent behaviour i n replacement s e r i e s a n a l y s i s , .e.g. between Holcus lanatus and T r i f o l i u m repens ( F i g . 16c, Chapter 5). ' I t i s important to understand how t h i s d i f f e r s from the frequency dependent competition d i s c u s s e d above. With niche d i f f e r e n t i a t i o n , i n t r a s p e c i f i c c o m p e t i t i v e pressure i s not the same as i n t e r s p e c i f i c c o m p e t i t i v e pressure but t h i s i s because each s p e c i e s always s u f f e r s more from c o m p e t i t i v e i n t e r a c t i o n with an i n d i v i d u a l of the same sp e c i e s than with an i n d i v i d u a l of the other s p e c i e s . In t h i s s i t u a t i o n there are no f o r c e s which should produce o s c i l l a t i o n s i n dominance and there i s no reason to invoke an e x p l a n a t i o n 214 i n v o l v i n g any ge n e t i c a l t e r a t i o n s of i n t r a - versus i n t e r - s p e c i f i c c o m p e t i t i v e a b i l i t y . Competition i s frequency dependent here because i n t r a s p e c i f i c c ompetition i s more intense than i n t e r s p e c i f i c c ompetition i n both s p e c i e s (because of more niche o v e r l a p between i n t r a s p e c i f i c i n d i v i d u a l s ) . Competition i s frequency dependent i n Pimentel's g e n e t i c feedback model because i n t r a s p e c i f i c c o m petition i s of a d i f f e r e n t nature than i n t e r s p e c i f i c c o m petition (because b e h a v i o u r a l i n t e r f e r e n c e e f f e c t s are d i f f e r e n t between i n t r a s p e c i f i c i n d i v i d u a l s than between i n t e r s p e c i f i c i n d i v i d u a l s ) . In Pimentel's g e n e t i c feedback mechanism, the same s e l e c t i o n that favours c h a r a c t e r s important i n i n t r a s p e c i f i c c o m p e t i t i o n , operates a g a i n s t c h a r a c t e r s important i n i n t e r s p e c i f i c c o m p e t i t i o n , and v i c e v e r s a . In. s e l e c t i o n f o r co m p e t i t i v e combining a b i l i t y , c o m p e t i t i v e a b i l i t y i s improved a c c o r d i n g to the p r e v a i l i n g c o m p e t i t i v e p r e s s u r e s , i n t r a - o_r i n t e r - s p e c i f i c , and no assumption i s imposed that c h a r a c t e r s important i n i n t r a - s p e c i f i c c o m petition need c o n f l i c t with , or even be any d i f f e r e n t from, c h a r a c t e r s . important i n i n t e r s p e c i f i c . c o m p e t i t i o n . 3) Di f f e r e n t l i m i t i n g f a c t o r s Another mechanism that has been proposed f o r the 'balance' of more than one s i m i l a r s p e c i e s i n c l o s e p r o x i m i t y occurs when the d e n s i t y of each s p e c i e s i s l i m i t e d by a d i f f e r e n t independent f a c t o r , or by the same f a c t o r at d i f f e r e n t times (Harper et a l . 1961). Braakhekke (1980) reviews r e l e v a n t recent l i t e r a t u r e . T h i s idea was r e f e r r e d to e a r l i e r with 215 regard to such l i m i t i n g f a c t o r s as drought t o l e r a n c e , pathogen or predator r e s i s t e n c e , e t c . Such f a c t o r s may suppress otherwise s u p e r i o r competitors or keep p o p u l a t i o n d e n s i t i e s of a l l competitors low enough so that resources are not u s u a l l y l i m i t i n g and hence competition r a r e l y ever occurs (Grime 1979). I t i s necessary however to draw a t t e n t i o n to i n s t a n c e s where resources are the d i f f e r e n t i a l l i m i t i n g f a c t o r s o p e r a t i n g as i s o f t e n purported to be the case under t h i s mechanism. Grubb (1977) giv e s the f o l l o w i n g examples: "...on some s o i l s n i t r o g e n i s the primary l i m i t i n g mineral f o r grasses, but phosphorous and potassium l i m i t the growth of legumes (Thurston 1969); on other s o i l s phosphorus may be the primary l i m i t i n g m i n e r a l f o r grasses and n i t r o g e n f o r sedges ( W i l l i s 1963). S i m i l a r l y the root growth of one s p e c i e s may be best i n s o i l pores of a c e r t a i n s i z e and that of another s p e c i e s may be best i n adjacent pores of a d i f f e r e n t s i z e (Sheikh & Rutter 1969)". Tilman (1977) found i n experiments with two sp e c i e s of freshwater algae that when both s p e c i e s were l i m i t e d by phosphate, one s p e c i e s was c o m p e t i t i v e l y dominant over the other, whereas when both s p e c i e s were l i m i t e d by s i l i c a t e , the second s p e c i e s was c o m p e t i t i v e l y dominant. At in t e r m e d i t e l e v e l s of phosphate and s i l i c a t e , each s p e c i e s was l i m i t e d by a d i f f e r e n t resource and s t a b l e c o e x i s t e n c e o c c u r r e d . It i s apparent that t h i s m e c h a n i s m i s at l e a s t p a r t i a l l y i n t e r p r e t e d i n terms of niche d i f f e r e n c e s . C l e a r l y , i f one sp e c i e s i s l i m i t e d much more by one f a c t o r than i s another s p e c i e s then the niche space w i t h i n which each s p e c i e s i s capable of success, even i n the absence of competitors, w i l l be 216 d i f f e r e n t . For example, grasses may be l i m i t e d by n i t r o g e n i n a community while legumes may not because the l a t t e r have symbiotic n i t r o g e n f i x i n g b a c t e r i a . Here, the legumes have a d i f f e r e n t source (or niche) f o r n i t r o g e n . In o v e r l a p p i n g niche space however, where com p e t i t i o n occurs f o r the same resource u n i t s , c o e x i s t e n c e r e q u i r e s another e x p l a n a t i o n . The p r e v a i l i n g environment in a p a r t i c u l a r h a b i t a t w i l l determine the degree of r i c h n e s s or l i m i t a t i o n of p a r t i c u l a r resources f o r a p a r t i c u l a r s p e c i e s . If the p o p u l a t i o n s i z e of each of two s p e c i e s i s l i m i t e d by two d i f f e r e n t l i m i t i n g resources i n a given h a b i t a t , t h i s i s not i n i t s e l f a s u f f i c i e n t c o n d i t i o n f o r c o e x i s t e n c e . Coexistence hinges on the c o n d i t i o n of balanced c o m p e t i t i v e a b i l i t i e s . For example, each s p e c i e s might be a b e t t e r competitor f o r one of the two resources (but not the same one) r e q u i r e d by both s p e c i e s . In t h i s sense the two s p e c i e s would have c o m p e t i t i v e combining a b i l i t y . If one of the s p e c i e s i s a b e t t e r competitor f o r both resources, c o e x i s t e n c e w i l l not be p o s s i b l e i n s p i t e of the ..fact that the p a r t i c u l a r h a b i t a t , f e a t u r e s a d i f f e r e n t l i m i t i n g resource f o r each s p e c i e s . ' D i f f e r e n t l i m i t i n g f a c t o r s ' f o r two s p e c i e s i s i n t e r p r e t e d by van den Bergh & Braakhekke (1978) as a form of ' f u n c t i o n a l ' niche d i f f e r e n t i a t i o n i n the sense of E l t o n (1927). In the above example, t h i s i s taken to mean that each s p e c i e s has a d i f f e r e n t s a t u r a t i o n l e v e l f o r both resources, i . e . i t i s capable of u s i n g d i f f e r e n t amounts of each resource (e.g. see e x t e n s i v e review by Andrew and Johansen 1978). T h i s combined with the a v a i l a b l e supply d i c t a t e s that one resource i s l i m i t i n g 217 for one s p e c i e s and the other resource i s l i m i t i n g f o r the other s p e c i e s . If however they are e x p l o i t i n g the same u n i t s of resource, any p o t e n t i a l f o r c o e x i s t e n c e must be measured a g a i n s t any d i f f e r e n t i a l c a p a c i t y to reduce the a v a i l a b i l i t y of these resource u n i t s to the other, not simply the occurrence of d i f f e r e n t l i m i t i n g f a c t o r s . Coexistence here i s not e x p l a i n e d by any (e.g. f u n c t i o n a l ) niche d i f f e r e n t i a t i o n , but by balanced c o m p e t i t i v e a b i l i t i e s i n the same ni c h e . Furthermore, balanced o v e r a l l c o m p e t i t i v e a b i l i t y can a l s o be maintained j u s t as w e l l by s e l e c t i o n f o r co m p e t i t i v e combining a b i l i t y when both s p e c i e s are l i m i t e d by the same resource; each s p e c i e s maintains a c e r t a i n , but not i n v a r i a b l e , suite, of a t t r i b u t e s (e.g. Table 1, Chapter 1) (perhaps d i f f e r e n t from the other) that permit i t to reduce, to the same extent i n each s p e c i e s , the a v a i l a b i l i t y of that resource to the other. 218 NICHE DIFFERENTIATION VERSUS COEVOLUTION OF COMPETITORS; ALTERNATIVE EVOLUTIONARY SOLUTIONS FOR SPECIES COEXISTENCE S e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y r e p r e s e n t s a mechanism by which competitors can be c o n s i d e r e d to co-evolve. U n l i k e the mechanism of niche d i f f e r e n t i a t i o n where n a t u r a l s e l e c t i o n ( o p e r a t i n g l a r g e l y on the weaker competitor) r e s u l t s in an escape from - or e l i m i n a t i o n of - the i n t e r a c t i o n between the s p e c i e s and thus allows c o e x i s t e n c e , a process of c o e v o l u t i o n must s a t i s f y two c r i t e r i a : a) The i n t e r a c t i o n i s preserved by n a t u r a l s e l e c t i o n (and c o e x i s t e n c e i s t h e r e f o r e i m p l i e d ) . T h i s f o l l o w s t r a d i t i o n i n a l l c l a s s i c contexts of c o e v o l u t i o n , e.g. as i n predator/prey, h o s t / p a r a s i t e , p o l l i n a t o r / h o s t p l a n t , e t c . . b) The g e n e t i c c o n s t i t u t i o n of both i n t e r a c t i n g p o p u l a t i o n s must be a d j u s t e d i n response to r e c i p r o c a l s e l e c t i o n pressure on each other (Janzen 1980). Leon (1974) and L e v i n & Udovic (1977) use c o e v o l u t i o n i n t h i s context with r e f e r e n c e to frequency-dependent models of c o m p e t i t i v e balance based on Pimentel's (1968) model of g e n e t i c feedback. Contrary to t h i s c ontext, others have regarded the c o e v o l u t i o n of competitors as niche (or c h a r a c t e r ) displacement (e.g. Lawlor & Maynard Smith 1976, Roughgarden 1976, Case 1979, C o n n e l l 1980). According to Slobodkin (1961, p.122), " i f two s p e c i e s p e r s i s t i n a p a r t i c u l a r r e g ion i t can be taken as axiomatic that some e c o l o g i c a l d i s t i n c t i o n must e x i s t between them". The d i f f e r e n c e s between s p e c i e s has become a u n i v e r s a l touchstone f o r s t u d i e s of s p e c i e s c o e x i s t e n c e . I n v e s t i g a t i o n s r e p e a t e d l y 219 and almost a u t o m a t i c a l l y e n t a i l a search f o r , or an i n t e r p r e t a t i o n i n v o l v i n g , some 'important' d i f f e r e n c e i n niche between c o e x i s t i n g competitors as though i t were some how remarkable to have found that one e n t i r e p o p u l a t i o n has not c o m p e t i t i v e l y excluded the other (e.g. as i n the c l a s s i c "paradox of the plankton" (Hutchinson 1961)). T h i s i s r e f l e c t i v e of the inadequate t y p o l o g i c a l view of the sp e c i e s i n which e c o l o g i c ideology i s embedded. T r a d i t i o n a l t h i n k i n g t r e a t s the s p e c i e s as more of a ta x o n o m i c a l l y d i s t i n c t i v e u n i t than as a wide ranging c o l l e c t i o n of e c o l o g i c a l l y d i f f e r e n t i n d i v i d u a l s and there i s much c u r r e n t i n t e r e s t by some to change t h i s view (Antonovics 1976a, Raven 1976, Harper 1982). Although t a x o n o m i c a l l y d i s t i n c t , two s p e c i e s with c o m p e t i t i v e combining a b i l i t y represent a dynamic continuum of g e n e t i c v a r i a t i o n i n respect to t h e i r c o m p e t i t i v e r e l a t i o n s h i p . The c r u c i a l c o n t e s t in s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y i s not 'species p i t t e d a g a i n s t s p e c i e s ' , but ra t h e r 'genotype a g a i n s t genotype', and any genotype may be co n f r o n t e d at d i f f e r e n t times with both i n f e r i o r and s u p e r i o r genotypes from e i t h e r taxonomic type. The present data showing r e c i p r o c a l i n t e r a c t i o n s between d i f f e r e n t n a t u r a l genotype p a i r s of Lolium perenne and T r i f o l i u m repens ( F i g s . 18 and 19, Chapter 6) i s the f i r s t demonstration of t h i s phenomenon of 'genotype versus genotype' s p e c i a l i z a t i o n . The r e l a t i v e c o m p e t i t i v e a b i l i t i e s of i n d i v i d u a l s f o r con t e s t e d resources ( r e g a r d l e s s of s p e c i e s ) appears to converge by s e l e c t i o n i n t o a common pool of a l t e r n a t i v e s t r a t e g i e s d e f i n e d by dynamic ' a t t r i b u t e complexes' which cannot be c a t e g o r i z e d on a taxonomic b a s i s . To the taxonomist or biogeographer concerned 220 with community composition, the s p e c i e s i s the r e l e v a n t u n i t of d i v e r s i t y here. To the e v o l u t i o n a r y e c o l o g i s t however concerned with community processes and dynamics, the o p e r a t i o n a l u n i t becomes the genotype and the p o p u l a t i o n and community are welded i n t o a s i n g l e e v o l u t i o n a r y arena ( i . e . F i g . 9, Chapter 3). The q u e s t i o n of c o e x i s t e n c e between competitors i s c e r t a i n l y v a l i d , but the i n s i s t e n c e that some d i f f e r e n c e i n niche or otherwise i s the cause i s not only a p r i o r i , but i t c o n t r a d i c t s the f a c t t h a t t h e i r c o e x i s t e n c e r e q u i r e s that they have f e a t u r e s i n common that render them both adapted to the same h a b i t a t ( c . f . Harper 1982). In s e l e c t i o n f o r co m p e t i t i v e combining a b i l i t y , the presence of (or the p o t e n t i a l to c o n t i n u o u s l y generate) r e l e v a n t g e n e t i c v a r i a n t s with c h a l l e n g i n g r e l a t i v e c o m p e t i t i v e a b i l i t y i n each s p e c i e s permits a s t a b l e c o e x i s t e n c e i n a preserved i n t e r a c t i o n that i s maintained by p e r s i s t e n t r e c i p r o c a l s e l e c t i o n ( c o e v o l u t i o n ) . T h i s process of competitors ' t r a c k i n g each other' i n v o l v i n g mutual s h i f t s i n gene f r e q u e n c i e s , i s not u n l i k e that i n t h e o r i e s f o r the e v o l u t i o n of c o e x i s t e n c e i n predator/prey (e.g. Pimentel 1961, Rosenzweig 1973), p l a n t / h e r b i v o r or p o l l i n a t o r ( E h r l i c h & Raven 1964, G i l b e r t & Raven 1975), h o s t / p a r a s i t e (Pimentel et a l . 1963), and model/mimic (Sheppard 1975) systems. Because a c o m p e t i t i v e i n t e r a c t i o n i s preserved, any o v e r l a p i n niche requirements may a l s o be preserved; there i s no need to assume any ambiguous c o n j e c t u r e that some d i f f e r e n c e i n niche requirements between s p e c i e s (however s u b t l e ) must e x p l a i n t h e i r c o e x i s t e n c e . The 'paradox' of c o i n c i d e n t s p e c i e s with l i t t l e apparent niche d i f f e r e n t i a t i o n need not be a paradox at 221 a l l . Moreover, s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y p r o v i d e s r e c o n c i l i a t i o n of the apparent paradox between "convergent a d a p a t a t i o n " to a common environment and "divergent a d a p t a t i o n " to other members of the community. If s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y were important i n a community of p l a n t s , one might expect there to be fewer temporary s i g n i f i c a n t d e v i a t i o n s from random a s s o c i a t i o n of s p e c i e s with time, i . e . i n o l d e r communities (step 3A i n the model of F i g u r e 9). T h i s i s s u b s t a n t i a t e d by the present data from c o n t a c t sampling (Table 12, Chapter 3). It i s commonly b e l i e v e d t h a t , " . . . i t i s always advantageous f o r e i t h e r p a r t y i n a c o m p e t i t i v e i n t e r a c t i o n to a v o i d the other whenever p o s s i b l e " (Pianka 1976). T r a d i t i o n a l theory assumes that .niche divergence i s to be expected between i n t e r s p e c i f i c c ompetitors whenever there i s r e l e v a n t g e n e t i c and environmental- v a r i a t i o n p e r m i t t i n g them to do so. T h i s seems i n t u i t i v e l y reasonable i n contexts of c o m p e t i t i o n i n v o l v i n g i n t e r s p e c i f i c i n t e r f e r e n c e s i n c e i t i s p o s s i b l e f o r i n t e r f e r e n c e e f f e c t s to be f e l t by i n t e r s p e c i f i c ., competitors and not i n t r a s p e c i f i c c o m p e t i t o r s . I t i s a l s o reasonable to expect that an i n f e r i o r competitor, when faced with a h i g h l y s u p e r i o r competitor (of any type) would evolve a niche s h i f t i f 'another' niche i s a v a i l a b l e and i f the consequence i s a reduced i n t e r a c t i o n . There are two important q u a l i f i c a t i o n s however to c o n s i d e r , e s p e c i a l l y with p l a n t s : 1) 'another' niche may not be as r e a d i l y a v a i l a b l e as with animals, s i n c e resources f o r p l a n t s do not appear as r e a d i l y p a r t i t i o n a b l e ; and 2) competition i s o f t e n predominantly of the ' e x p l o i t a t i o n ' type and, i n c o n t r a s t to i n t e r f e r e n c e 222 e f f e c t s , e x p l o i t a t i o n e f f e c t s w i l l always be f e l t by both i n t e r - and i n t r a - s p e c i f i c c ompetitors. The consequence of 1) i s t h a t , i f c o m p e t i t i v e e x c l u s i o n i s t h r e a t e n i n g , s e l e c t i o n may favour c o m p e t i t i v e combining a b i l i t y i n s t e a d of niche d i f f e r e n t i a t i o n . The consequence of 2) i s that i f e x p l o i t a t i o n c o m p e t i t i v e a b i l i t i e s are very s i m i l a r ( e i t h e r c o i n c i d e n t a l l y or a f t e r s e l e c t i o n ) , there i s no reason that subsequent s e l e c t i o n should u n e q u i v o c a l l y l e a d to niche divergence between the two p o p u l a t i o n s , even i f such o p p o r t u n i t i e s f o r divergence were (or became) abundant. The reason f o r t h i s f o l l o w s from c o n s i d e r a t i o n of the s e l e c t i v e f o r c e s o p e r a t i n g on i n d i v i d u a l s i n such i n s t a n c e s . If niche divergence were to take p l a c e , both p l a n t p o p u l a t i o n s would s t i l l have to contend with i n t r a s p e c i f i c e x p l o i t a t i o n competition which i n t h i s case i s no d i f f e r e n t i n i t s e f f e c t on i n d i v i d u a l f i t n e s s than i n t e r s p e c i f i c c o m p e t i t i o n . I n d i v i d u a l s of one s p e c i e s , say A which happen to ' f i n d ' a niche that i s not e x p l o i t e d by s p e c i e s B would not be expected to leave any more descendents than those i n d i v i d u a l s of A e x p l o i t i n g the same niche as B. The consequence would be niche expansion f o r A, not divergence from B, and does not even n e c e s s a r i l y r e q u i r e the presence of B. Any advantage f o r s p e c i e s A here then has nothing to do with avoidance of s p e c i e s B. A q u e s t i o n which presents i t s e l f from the above c o n s i d e r a t i o n s i s the f o l l o w i n g : How do i n t r a s p e c i f i c competitors c o e x i s t ? The answer i s almost t r i v i a l and i t i s p r e c i s e l y t h i s f a c t that presents f a r - r e a c h i n g i m p l i c a t i o n s . I n t r a s p e c i f i c competitors c o e x i s t by and l a r g e because r e l a t i v e 223 c o m p e t i t i v e a b i l i t i e s are maintained roughly balanced amongst the members of the p o p u l a t i o n through n a t u r a l s e l e c t i o n . The members of a p o p u l a t i o n do not 'decide' to d i v e r g e from another p o p u l a t i o n , and in c o n t r a s t , maintain a 'balance' w i t h i n t h e i r own. An i n d i v i d u a l w i l l leave more descendents, i f those descendents are b e t t e r at ' a v o i d i n g c o m p e t i t i v e i n t e r a c t i o n ' or are ' more s u c c e s s f u l i n c o m p e t i t i v e i n t e r a c t i o n ' r e g a r d l e s s of the taxonomy of t h e i r p o t e n t i a l c o n t e s t a n t s . The former mechanism however i s r e s t r i c t i v e as i t i s only r e l e v a n t i n e x p l a n a t i o n s of the c o e x i s t e n c e of d i f f e r e n t taxonomic u n i t s ; an i n d i v i d u a l can r a r e l y a v o i d competition with members of i t s own s p e c i e s i n a resource l i m i t e d environment. S e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y however has broader a p p l i c a t i o n as i t e x p l a i n s the c o e x i s t e n c e of i n d i v i d u a l s , i r r e s p e c t i v e of taxonomy. A c c o r d i n g l y , i f we suppose that two s p e c i e s have broadly o v e r l a p p i n g niches and compete l a r g e l y by ' e x p l o i t a t i o n ' mechanisms, we should not expect strong s e l e c t i o n f o r niche divergence i f these s p e c i e s have c o m p e t i t i v e combining a b i l i t y . Niche divergence a f f o r d s l i t t l e advantage . because the two p o p u l a t i o n s behave c o m p e t i t i v e l y as a ' s i n g l e s p e c i e s ' to begin with. Since, i n both p o p u l a t i o n s , o v e r a l l i n t e r - and i n t r a - s p e c i f i c c o m p e t i t i v e a b i l i t i e s are v i r t u a l l y e q u i v a l e n t i n t h e i r e f f e c t s , niche divergence would do nothing to change the s e v e r i t y of c o m p e t i t i v e p r e s s u r e s on i n d i v i d u a l s in e i t h e r p o p u l a t i o n . One would no more expect divergence between two such p o p u l a t i o n s i n response to i n t e r s p e c i f i c c o m petition than one would expect divergence w i t h i n e i t h e r p o p u l a t i o n i n response to i n t r a s p e c i f i c c o m p e t i t i o n . 224 T h e o r e t i c a l c o n s i d e r a t i o n s using the s u b s t i t u t i v e replacement s e r i e s model of c o m p e t i t i o n (de Wit 1960, 1961, 1970 van den Berghe & Braakhekke 1978) have shown that a s t a b l e e q u i l i b r i u m of two s p e c i e s i s p o s s i b l e only under frequency dependent r e g u l a t i o n , u s u a l l y thought to r e f l e c t niche d i f f e r e n c e s . In c o n t r a s t , there need be no d i f f e r e n c e between the o v e r a l l hardships experienced from i n t r a - or i n t e r - s p e c i f i c competitors i n c o e x i s t i n g s p e c i e s with c o m p e t i t i v e combining a b i l i t y . I t i s a commonly h e l d no t i o n that i n s p e c i e s with l i t t l e niche d i f f e r e n t i a t i o n and roughly balanced c o m p e t i t i v e a b i l i t i e s , the f r e q u e n c i e s of the two p o p u l a t i o n s w i l l e x h i b i t l a r g e u n p r e d i c t a b l e f l u c t u a t i o n s i n a s t o c h a s t i c environment, which s t r o n g l y i n c r e a s e s the r i s k of e x t i n c t i o n of one of the two s p e c i e s (Berendse 1981b). A q u a l i f y i n g p o i n t however must be noted. E x t i n c t i o n from c o m p e t i t i o n occurs when an i n f e r i o r competitor i s faced with a much s u p e r i o r competitor. E x t i n c t i o n due to c o m p e t i t i v e e x c l u s i o n may occur i f changes in the environment produces changes i n the r e l a t i v e c o m p e t i t i v e a b i l i t i e s of the s p e c i e s , but not i f the weaker competitor can respond by s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y . P i g o t t (1982) d i s c u s s e s examples of c o m p e t i t i v e e x c l u s i o n in v e g e t a t i o n in response to environmental changes, but most i n v o l v e a r t i f i c i a l treatments of f e r t i l i z e r a p p l i c a t i o n and do not r e f l e c t a l l of the ways that s p e c i e s respond to t h e i r b i o t i c environment a g a i n s t the v a r i a b l e background of s t o c h a s t i c events in nature. A f u l l y adequate understanding of s p e c i e s c o e x i s t e n c e r e q u i r e s a g r e a t e r emphasis on the v a r i a b i l i t y w i t h i n s p e c i e s 225 with r e s p e c t to both fundamental niche requirements and r e l a t i v e c o m p e t i t i v e a b i l i t i e s , concurrent with an awareness of the d i f f e r e n c e s between s p e c i e s . S e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y and c o m p e t i t i v e combining a b i l i t y are l i k e l y to operate in c o n c e r t i n nature and together d e s c r i b e the c o n t r i b u t i o n of n a t u r a l s e l e c t i o n i n the model f o r community e v o l u t i o n proposed in Chapter 3 ( F i g . 9). E f f o r t s are needed to r e v e a l how and under what circumstances n a t u r a l s e l e c t i o n a f f e c t s each of these two components and when i t i s l i k e l y to a f f e c t one more than the other. Whether s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y ( F i g . 21-B) or s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y ( F i g . 21- C) or both d e p i c t s the e v o l u t i o n a r y route l e a d i n g to c o e x i s t e n c e of two s p e c i e s (or whether c o m p e t i t i v e e x c l u s i o n i s the outcome as per Gause's p r i n c i p l e - F i g . 21-A) w i l l depend on at l e a s t four f a c t o r s : 1) The i n i t i a l magnitude of d i f f e r e n c e (or extent of s i m i l a r i t y ) in both fundamental niche requirements and c o m p e t i t i v e a b i l i t y upon f i r s t encounter . For example, i f the niches of two s p e c i e s o v e r l a p e x t e n s i v e l y and one i s a f a r s u p e r i o r competitor than the other, then i t i s reasonable to expect that c o m p e t i t i v e e x c l u s i o n w i l l be v i r t u a l l y immediate. S e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y i s u n l i k e l y i n cases i n which d i f f e r e n c e s i n c o m p e t i t i v e a b i l i t i e s are l a r g e . 226 2) The a v a i l a b i l i t y of (or p o t e n t i a l to generate) d i f f e r e n t ' r e l e v a n t ' g e n e t i c v a r i a n t s p e r t a i n i n g to fundamental niche requirements versus r e l a t i v e c o m p e t i t i v e a b i l i t y . Some s p e c i e s may have more g e n e t i c v a r i a t i o n r e l a t e d to r e l a t i v e c o m p e t i t i v e a b i l i t y than to fundamental niche requirements, and v i c e v e r s a . T h i s w i l l be l a r g e l y determined by the nature and magnitude of the a v a i l a b l e gene pool and the means of propagation in the two s p e c i e s ( i . e . autogamy, allogamy, apomixis, c l o n a l spread). O u t c r o s s i n g s p e c i e s w i l l g e n e r a l l y have g r e a t e r p o t e n t i a l f o r ge n e r a t i n g adaptive g e n e t i c v a r i a n t s and t r a c k i n g changes i n the b i o t i c environment than w i l l i n c r o s s i n g or asexual s p e c i e s ( S o l b r i g 1976, Glesener & Tilman 1978). 3) The range of resource space a v a i l a b l e i n the p r e v a i l i n g environment .for niche expansion or displacement . Novel niche space may be u n a v a i l a b l e i n a c e r t a i n h a b i t a t or s p e c i e s may be c o n s t r a i n e d by a matrix of nearby p o t e n t i a l competitors so that the o p t i o n of " e v o l v i n g - t o - a v o i d - c o m p e t i t i o n " i s not f e a s i b l e (Krebs 1979). S e l e c t i o n may i n s t e a d favour c o m p e t i t i v e combining a b i l i t y i n such circumstances. T h i s . a p p l i e s p a r t i c u l a r l y to p l a n t s which, being mostly s e s s i l e , r e q u i r e a s u f f i c i e n c y of resources v i r t u a l l y i n t h e i r immediate v i c i n i t y , r esources which are not l a r g e l y packaged i n d i s c r e t e p a r t i t i o n a b l e u n i t s (Harper 1965, 1968), and the same resources that neighbours are a l s o f o r c e d to make demands on. 227 4) The presence (or absence) of p o s i t i v e r e l a t i o n s h i p s o c c u r r i n g concomitantly with competition between two s p e c i e s . It i s not d i f f i c u l t to imagine (but o f t e n overlooked) that i n d i v i d u a l s may compete in one sense and yet i n t e r a c t b e n e f i c i a l l y i n another sense (e.g. Agnew & Haines 1960, Mather 1961, Harper 1964, Story 1967, Putwain & Harper 1970, Walker et a l . 1972). A s u p e r i o r competitor may stand to 'loose out' i n the long run i f i t e l i m i n a t e s a s p e c i e s which has something p o s i t i v e to o f f e r . Commensalistic or m u t u a l i s t i c a s s o c i a t i o n s i n which co m p e t i t i v e a b i l i t i e s are roughly balanced may t h e r e f o r e be favoured (and maintained) by n a t u r a l s e l e c t i o n and s t a b l e c o e x i s t e n c e may r e s u l t . Moreover niche convergence may occur i f beneficence i s a strong enough s e l e c t i v e f o r c e . T h i s i s r e l e v a n t to the e x p l a n a t i o n of c o m p e t i t i v e i n t e r a c t i o n s between Lolium perenne and T r i f o l i u m repens demonstrated in the present study ( F i g . 16, Chapter 5; Chapter 6). Cooperation w i t h i n a s i n g l e outbreeding p o p u l a t i o n w i l l be necessary f o r s u c c e s s f u l mating. For example, in d i o e c i o u s s p e c i e s , or h e t e r o s t y l i c s p e c i e s (e.g. primrose) the 'male', and 'female''(or 'pin' and 'thrum' in primrose) w i l l compete with each other f o r r e s o u r c e s . But i n regards to r e p r o d u c t i o n they w i l l r e q u i r e a c o o p e r a t i v e r e l a t i o n i n order to leave any descendents at a l l . Mather (1961) p o i n t s out that i n such i n s t a n c e s s e l e c t i o n w i l l favour more e q u i v a l e n t c o m p e t i t i v e power and t h i s may i n v o l v e not only an improvement i n c o m p e t i t i v e a b i l i t y of the i n f e r i o r form, but a l s o a r e l a t i v e r e d u c t i o n i n the a g g r e s s i v e n e s s of the s u p e r i o r form. T h i s i s s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y and c o u l d occur not 228 only w i t h i n a s i n g l e s p e c i e s as above but a l s o between two d i f f e r e n t s p e c i e s which i n t e r a c t i n some other b e n e f i c i a l manner and yet a l s o compete f o r resources (e.g. Chapter 6). A p l a n t s p e c i e s may exert a p o s i t i v e (enhancing) e f f e c t on another p l a n t s p e c i e s i n a number of p o s s i b l e ways. Furthermore, i n a mutualism, the b e n e f i t gained by one s p e c i e s may not be the same kind of b e n e f i t gained by i t s a s s o c i a t e . P o s s i b l e b e n e f i c i a l i n t e r a c t i o n s i n p l a n t s i n c l u d e : a) P r o v i d i n g m a t e r i a l s i n l i m i t e d supply (e.g. a legume may p r o v i d e n i t r o g e n to a non-legume). b) P r o v i d i n g p h y s i c a l p r o t e c t i o n from c l i m a t e ( M a r s h a l l 1967), p r e d a t o r s (Harper & Sagar 1953), p e s t s and d i s e a s e s ( A t s a t t & O'Dowd 1976). c) P r o v i d i n g p h y s i c a l support f o r growth (e.g. epiphytes) or a g a i n s t l o d g i n g (Trenbath 1976). d) P r o v i d i n g s p e c i a l environmental c o n d i t i o n s (e.g. shade or high humidity) fav o u r a b l e f o r germination and establishment (Harper 1964). e) Encouragement of favourable rhizosphere components. or discouragement of d e t r i m e n t a l ones ( C h r i s t i e et a l . 1974). f) L i b e r a t i o n of s t i m u l a t i n g chemicals (Roy 1960, Tukey 1970). g) D i s p e r s a l with commercial seeds and p r o t e c t i o n from d e s t r u c t i o n by man by mimicing the crop p l a n t (Wickler 1968). 229 CONCLUSIONS D i f f e r e n c e i n niche requirements w i l l c e r t a i n l y e x p l a i n why some sp e c i e s are able to c o e x i s t . Much evidence, mostly from s t u d i e s on animals (e.g. see Cody 1974, Schoener 1974, Diamond 1978), tend to "... support a d e r i v a t i v e hypothesis of Darwinism t h a t , i n a p o p u l a t i o n of a s p e c i e s , mechanisms which reduce competition between i t and p o p u l a t i o n s of other s p e c i e s tend to p e r s i s t " ( G i l b e r t et a l . 1952, p. 310). To i n s i s t t h a t t h i s i s some s o r t of law or axiom however i s 'Gaussian' i n o r i g i n and t r a d i t i o n and p a t e n t l y c o n t r a d i c t s the enormous amount of s p e c i e s d i v e r s i t y found e s p e c i a l l y i n many types of p l a n t communities. P l a n t s , i n c o n t r a s t to animals, have v i r t u a l l y nothing comparable to the 'food n i c h e ' . P l a n t e c o l o g i s t s are t h e r e f o r e l a r g e l y unable to adopt most of the models and t h e o r i e s which are ap p a r e n t l y adequate i n e x p l a i n i n g animal c o e x i s t e n c e . In s p i t e of t h i s , some p l a n t e c o l o g i s t s have attempted to account f o r p l a n t s p e c i e s c o e x i s t e n c e i n nature, l a r g e l y on the b a s i s of niche d i f f e r e n t i a t i o n (e.g. Grubb 1977, van den Bergh'& Braakhekke 1978, Newman '1982). Co e v o l u t i o n by s e l e c t i o n f o r co m p e t i t i v e combining a b i l i t y p r o v i d e s an e v o l u t i o n a r y mechanism f o r c o e x i s t e n c e which does not u n e q u i v o c a l l y r e q u i r e that every s p e c i e s occupy a d i f f e r e n t niche i n a community, nor does i t depend on frequency-dependent s e l e c t i o n mediated by b e h a v i o u r a l i n t e r f e r e n c e phenomena. I t i s a c o e v o l u t i o n a r y theory of co m p e t i t i v e c o e x i s t e n c e in which the consequence of r e c i p r o c a l s e l e c t i o n means that d i f f e r e n t s p e c i e s are p e r s i s t e n t l y making demands f o r the same l i m i t i n g r e s o u r c e s . 230 Moreover, given s u f f i c i e n t g e n e t i c v a r i a t i o n , there i s good reason to suspect that o p p o r t u n i t i e s f o r n a t u r a l s e l e c t i o n to improve the r e l a t i v e c o m p e t i t i v e a b i l i t y i n a p o p u l a t i o n of i n f e r i o r p l a n t competitors ( e.g. Table 1, Chapter 1) may be f a r more numerous than the o p p o r t u n i t i e s f o r niche d i f f e r e n t i a t i o n i n v e g e t a t i o n . T h i s idea has been overlooked (or dismissed) as a v i a b l e theory f o r s p e c i e s c o e x i s t e n c e because the meaning and d i s t i n c t i o n of 'competitive a b i l i t y ' have been misrepresented and the r o l e of n a t u r a l s e l e c t i o n in a d j u s t i n g i t has been underestimated a p r i o r i . Slobodkin (1968) reasoned that i f two s p e c i e s which appear to have the same ecology p e r s i s t together i n a uniform experimental environment, t h i s f i n d i n g does not d i s p r o v e Gause's hy p o t h e s i s ; rather i t proves that the organisms do not have the same ecology or that the environment was not uniform. T h i s f i n d i n g in f a c t does not even prove Slobodkin's c l a i m ; rather i t suggests that the two s p e c i e s may exert r e l a t i v e l y equal c o m p e t i t i v e pressures -against each other and that t h i s i s maintained by r e c i p r o c a l s e l e c t i o n , , i . e . they have competitive, combining a b i l i t y . Furthermore, even i f two s p e c i e s have niches which do not completely o v e r l a p , the a b i l i t y to c o e x i s t may be as much a consequence of s e l e c t i o n f o r c o m p e t i t i v e combining a b i l i t y i n the region of niche o v e r l a p as i t i s a consequence of s e l e c t i o n f o r e c o l o g i c a l combining a b i l i t y where competition can be avoided. 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